Rapid review: infectious disease

• This page is a revision tool, not a first lesson. Each block gives the presenting picture, the single most likely diagnosis, the best first test, the best first treatment, the commonest wrong answer, and the full MedGrasp topic to read.
• In infection the answer starts with the patient, the route, the place and the host: a returned traveller is malaria until proven otherwise; a hot swollen joint is septic arthritis until proven otherwise; meningism in an ill patient is meningitis until proven otherwise.
• The commonest killers in infection are sepsis, bacterial meningitis, severe malaria, tuberculous meningitis and neglected severe bacterial infection. Recognition and the first hour matter far more than the last.
• Most infections are diagnosed from a careful history, an examination repeated over days, and cheap tests: a blood film, a lumbar puncture, a gram stain, blood cultures and a chest radiograph.
• Empiric treatment should cover the likely organism, then be narrowed as soon as culture or molecular results allow. Broad-spectrum antibiotics are a safety net, not a answer.
• Latent, chronic and opportunistic infections need their own long game: the immune status, the geography, and the exposure matter as much as the organism.

By the end of this page you should be able to:

1. Run through the classic infectious disease presentations in examination order, naming the likely diagnosis, the first test, the first treatment and the common trap in each.
2. Recognise the red flags that turn a routine infection into an emergency.
3. Use the flowchart to decide whether a patient needs urgent decompression, urgent antibiotics, source control, or careful outpatient review.
4. Move confidently from each block to the full MedGrasp topic for depth.
5. Explain why a single negative test does not exclude malaria, sepsis, meningitis or tuberculosis.
6. Apply each block in a setting with limited tests.
7. State which antibiotics or antimalarials are dictated by guideline edition and geography.

Infectious disease presents as syndromes, not as organism lists. Block 1 below is undifferentiated prolonged fever; blocks 2 to 4 are life-threatening central and whole-body infection; blocks 5 to 7 are infection-specific patterns; blocks 8 to 11 are site, route and setting. Each block states the presenting picture in plain clinical words, the single most likely diagnosis, the best first test, the best first treatment, the commonest wrong answer, and the full topic link. Where guideline editions and thresholds matter, they are named.

This is a curriculum-gap revision page, so it builds only on the full topics it names. Conceptually it connects to every infectious disease topic on the site: a block is a one-page summary, and the full topic carries mechanism, depth and evidence. The approaches here follow recent Delphi consensus work on fever, recent hospital intensive care guidance on bacterial meningitis, the 2021 Surviving Sepsis Campaign, World Health Organization HIV and tuberculosis materials, CDC malaria treatment tables, the CDC Yellow Book 2026 for travel, CDC tuberculosis treatment guidance, a systematic review and meta-analysis of antibiotic therapy for chronic osteomyelitis, and NICE clinical guideline CG139 on healthcare-associated infection. ^{[1][2][3][4][5][6][7][8][9][10][11]}

A useful reflex before naming a disease is to ask which single test changes management: a blood film in malaria, a lumbar puncture and cerebrospinal fluid (CSF) examination in meningitis, blood cultures and lactate in sepsis, a fourth-generation antigen and antibody test in HIV, and a sputum sample or molecular test for tuberculosis. For bone infection the early test is magnetic resonance imaging (MRI); for a hot joint the test is urgent joint aspiration. ^[6][8][9]

Presenting picture: a patient with an illness lasting more than 3 weeks, documented temperatures above 38.3 degrees C on several occasions, in whom a standard minimum evaluation has already been completed and the cause is still unknown. In the 2024 Delphi consensus refinement, a modern classic case is an immunocompetent adult with fever for more than 3 weeks after the standard panel and imaging, and the same temperature threshold above 38.3 degrees C. Early HIV disease is usually excluded from the classic definition, although HIV-associated fever lasting more than 4 weeks as an outpatient or more than 3 days in hospital has its own category. ^[1]

Single most likely diagnosis: the cases that win are usually atypical presentations of common disease. In large series infection accounts for roughly one in two to one in five cases, malignancy about one fifth, non-infectious inflammatory disease about one tenth, miscellaneous causes of a similar size, and a quarter to half receive no final diagnosis; the exact proportions vary with geography, income level and the definition of fever used, so infection is proportionally higher in low- and middle-income settings and undiagnosed cases proportionally higher in high-income settings. Tuberculosis, infective endocarditis and cytomegalovirus and Epstein-Barr virus infection recur as the infective causes most often missed, while lymphoma and adult-onset Still disease recur as the common non-infective ones. ^[12]

Best first test: repeated history and examination looking for potential diagnostic clues, plus completion of the minimum standard panel if it is not finished: full blood count, electrolytes and liver function, erythrocyte sedimentation rate, C-reactive protein, ferritin, thyroid function, autoimmune serology, HIV serology, urinalysis, three sets of blood cultures, tuberculin skin test or interferon-gamma release assay, chest imaging, and ultrasonography or computed tomography of the abdomen. ^[1]

Best first treatment: there is no drug that treats the label. In a stable patient empiric antimicrobial or corticosteroid therapy should be avoided because it can mask the underlying illness; reserve it for neutropenic, immunocompromised or critically ill patients, and for selected situations such as suspected tuberculosis, brucellosis, tularemia or infective endocarditis. ^[1]

Commonest wrong answer: ordering exotic panels before the history and examination, missing a dental abscess, drug fever or tuberculosis, or using empiric steroids or antibiotics early so that the real disease becomes invisible.

Full topic: infectious-disease/fever-of-unknown-origin

Presenting picture: fever with headache, neck stiffness and photophobia, sometimes with reduced consciousness, seizures or a non-blanching rash. Bacterial meningitis in adults is a same-day emergency; the CSF classically looks cloudy with more than 1,000 white cells per microlitre, protein greater than 1 g per litre and glucose less than four tenths of the blood level, while viral disease is usually clearer with 5 to 1,000 cells, protein 0.4 to 1 g per litre and normal glucose. Fever and headache in a patient with a changed personality or a new seizure point towards encephalitis, in which memory, behaviour or consciousness is affected and the illness usually runs over hours to days. ^[2]

Single most likely diagnosis: in culture-positive adult community-acquired bacterial meningitis, pneumococcus is commonest, identified in about 60 percent of cases, followed by meningococcus in about 7 percent and Listeria in about 5.5 percent, with Listeria favouring older or immunocompromised adults. Neck stiffness accompanies about 69 to 83 percent. ^[2] In encephalitis in a previously healthy adult in a temperate country, herpes simplex virus type 1 (HSV-1) is the commonest cause that must be treated urgently, while autoimmune causes are now at least as common in many Western populations and must be investigated at the same time.

Best first test: two pairs of blood cultures and other baseline tests, then a lumbar puncture as soon as it is safe. Brain imaging before lumbar puncture is not routine: it is needed for new focal deficits, recent seizures, severely impaired consciousness, signs of raised intracranial pressure, immunocompromise, known central nervous system disease, cranial nerve abnormality, or older age depending on the guideline used; if imaging will delay treatment, antibiotics and dexamethasone come first. ^[2][3] Where herpes simplex encephalitis is suspected, start intravenous aciclovir 10 mg per kilogram 8-hourly empirically. The diagnosis then rests on cerebrospinal fluid polymerase chain reaction results, magnetic resonance imaging of the temporal lobes, and an electroencephalogram. HSV-1 detection can be falsely negative very early, so the liquor may need repeating after 2 to 3 days if suspicion remains high. ^[2]

Best first treatment: for suspected bacterial meningitis, start a third-generation cephalosporin, usually ceftriaxone 2 g intravenously 12-hourly, with ampicillin 2 g intravenously 4-hourly for Listeria, adding vancomycin where pneumococcal cephalosporin resistance is expected; where herpes simplex encephalitis is suspected, add intravenous aciclovir. Droplet precautions continue until meningococcus is excluded. ^[3] Contacts of a confirmed meningococcal case need prophylaxis within 24 hours: ciprofloxacin 500 mg orally once, ceftriaxone 250 mg intramuscularly once, or rifampicin 600 mg intravenously twice daily for 2 days. ^[2][18]

Commonest wrong answer: waiting for imaging before antibiotics when the patient is clinically unwell; treating meningococcal suspicion with oral, sulphonamide or macrolide drugs; assuming absence of the classic triad excludes the disease; or giving aciclovir late and then stopping it after one negative cerebrospinal fluid sample.

Full topics: infectious-disease/meningitis, infectious-disease/encephalitis

Presenting picture: infection with any organ dysfunction, that is a new change in mental state, respiratory rate 22 per minute or more, low systolic blood pressure, lactate above 2 mmol per litre, poor urine output or mottled skin. The patient may have fever, but may instead be cold and confused; an elderly patient may present only with a fall or anorexia. Pneumonia, urinary infection, intra-abdominal infection and skin and soft tissue infection together cause most of it.

Single most likely diagnosis: sepsis from a community or healthcare-acquired infection. Uncomplicated infection is not sepsis; sepsis is organ dysfunction produced by a dysregulated host response. ^[4] A rise in the Sequential Organ Failure Assessment (SOFA) score of 2 points or more from baseline confirms organ dysfunction. ^[4][19]

Best first test: lactate, two sets of blood cultures before antibiotics if it does not substantially delay treatment, full blood count and renal and liver function, urinalysis, chest radiograph and focused imaging of abdomen or pelvis when the source is unclear. ^[4]

Best first treatment: for possible septic shock, take cultures without adding a substantial delay of more than about 45 minutes, give broad-spectrum antibiotics within an hour, give at least 30 mL per kilogram of crystalloid within 3 hours if there is hypoperfusion or shock, and start noradrenaline first to maintain a mean arterial pressure (MAP) of 65 mm Hg or more. Hydrocortisone 200 mg a day as 50 mg intravenously every 6 hours is added when vasopressors are still required after 4 hours; steroids are for shock, not for sepsis without shock. Simultaneous source control (line removal, drainage, debridement, relief of obstruction) starts early and is reviewed daily. ^[4]

Wrong answers: using quick SOFA (qSOFA) as the definition rather than as a screen; waiting helplessly for a culture result in shock; giving starches or gels; starting steroids before shock; and failing to identify and control an abscess or an infected device.

Full topic: infectious-disease/sepsis

Presenting picture: an adult with fever, weight loss and chronic diarrhoea, or an unexplained severe or recurrent infection; or a young adult with an influenza-like acute retroviral illness 2 to 4 weeks after a risk exposure; or a person who has never been tested and now has an AIDS-defining illness such as Pneumocystis pneumonia, cerebral toxoplasmosis or cryptococcal meningitis. CD4 stage 3, that is AIDS, is a count below 200 cells per microlitre. ^[5]

Single most likely diagnosis: human immunodeficiency virus infection with its characteristic immune failure, usually at a CD4 count below 200 cells per microlitre for Pneumocystis, below 100 for toxoplasmosis, and below 50 for disseminated Mycobacterium avium complex; always ask about tuberculosis, which is the commonest serious co-infection in high-burden settings. Active and latent tuberculosis should be assessed before antiretroviral screening is complete. ^[5]

Best first test: a fourth-generation HIV antigen and antibody immunoassay, confirmed by a differentiation assay, with HIV RNA testing in the window period or after an indeterminate result; CD4 count, viral load, resistance testing and co-infection screens follow. ^[5]

Best first treatment: treat HIV for everyone at diagnosis, with an integrase inhibitor-based regimen preferred, provided tuberculosis meningitis (where antiretroviral therapy should not be started in the first 8 weeks according to CDC guidance, whereas WHO guidance is to delay it at least 4 weeks and start it within 8 weeks) and cryptococcal meningitis are not present; offer chemoprophylaxis against Pneumocystis, toxoplasmosis and Mycobacterium avium complex at the relevant CD4 counts, and against tuberculosis whenever latent infection is suspected. After 3 months of suppressive antiretroviral therapy above the threshold, prophylaxis can be stopped. ^[5][13][14]

Commonest wrong answers: diagnosing tuberculosis in an HIV-positive patient with "just a cough" and ordinary radiographs without bacteriology; starting oral antiretrovirals without considering drug-drug interactions such as rifampicin; and forgetting that partial immune recovery can paradoxically worsen an infection in the first months of treatment. ^[5]

Full topics: infectious-disease/hiv-infection, infectious-disease/hiv-prevention-prep-and-pep, infectious-disease/pneumocystis-pneumonia, infectious-disease/cryptococcal-meningitis, infectious-disease/toxoplasmosis

Presenting picture: an undifferentiated febrile illness in a person who has been in a malaria-endemic area within the previous 12 months, with chills or rigors, headache, myalgia and gastrointestinal symptoms, and an enlarged spleen. Ask about prophylaxis, exact destinations and night-time mosquito exposure rather than just the country. P. falciparum can bring fever within days, whereas P. vivax and P. ovale may appear up to a year later. Very low parasite densities can cause severe disease in a non-immune traveller, so one film is not enough.

Single most likely diagnosis: Plasmodium falciparum malaria, the most deadly species, recognisable when parasitaemia is 5 percent or more of red cells or when any organ criterion such as coma, renal injury, acidosis, hypoglycaemia, severe anaemia under 7 g per decilitre, shock or pulmonary oedema is present. Non-falciparum disease can look identical early.

Best first test: thick and thin blood films, repeated every 12 to 24 hours for up to three sets if the first is negative, with parasitaemia quantified every time. A rapid antigen test is useful when microscopy is not immediately available, but a blood film remains the standard and informs speciation and severity. G6PD testing must be arranged early whenever P. vivax or P. ovale is possible, because anti-relapse therapy follows the acute attack. ^[6][15]

Best first treatment: for severe malaria of any species, intravenous artesunate 2.4 mg per kilogram at 0, 12 and 24 hours, then once daily while parasitaemia stays above 1 percent, with a full oral follow-on course starting 8 to 24 hours after the last intravenous dose once parasitaemia is 1 percent or less and the patient can swallow. For uncomplicated P. falciparum, an artemisinin-based combination therapy such as artemether-lumefantrine is first line; for P. vivax or P. ovale, acute treatment with chloroquine where sensitive, or an artemisinin-based combination, followed by primaquine 30 mg base daily for 14 days, or tafenoquine 300 mg as a single dose in a well adult aged 16 years or older, after G6PD deficiency has been excluded. Primaquine and tafenoquine are contraindicated in pregnancy. ^[6]

Commonest wrong answers: excluding malaria because the traveller completed prophylaxis, diagnosing influenza from a normal initial blood film, missing hypoglycaemia or severe anaemia, or giving artesunate without planning the follow-on course.

Full topics: infectious-disease/malaria, infectious-disease/returning-traveller-with-fever

Presenting picture: one of four patterns. Urethral or cervical discharge in an at-risk adult. A single or multiple genital ulcer. Lower abdominal or pelvic pain with fever. An acute swollen joint with migratory arthritis and tenosynovitis in a sexually active young adult. Take a sexual history, examine for a rash, and remember that co-infection is the rule rather than the exception.

Single most likely diagnosis: the syndrome rather than one organism. Urethral or cervical discharge is usually chlamydia or gonorrhoea; a painless genital ulcer suggests syphilis while a painful ulcerated lesion suggests herpes; lower abdominal pain with pelvic tenderness suggests pelvic inflammatory disease from an ascending infection; urethritis in a man with epididymo-orchitis and sterile pyuria suggests a urethral or gonococcal focus. Disseminated gonococcal infection may present as a rash with joint pain.

Best first tests: nucleic acid amplification tests (NAATs) from the affected anatomical site and from any exposed site; a gram stain and culture where gonorrhoea is suspected, for antimicrobial susceptibility; syphilis serology even when another infection is obvious; and an HIV test including a re-test after the window period. Urine NAAT is convenient, but it is not enough on its own if pharyngeal or rectal exposure has occurred. ^[5]

Best first treatment: syndrome-led empiric cover for chlamydia and gonorrhoea while awaiting results; parenteral ceftriaxone and doxycycline as an empiric framework for pelvic inflammatory disease; and aciclovir for herpes. Treat partners, offer HIV post-exposure prophylaxis when an exposure is within 72 hours, and offer HIV pre-exposure prophylaxis where ongoing risk exists. The exact regimen and dose must follow the current national sexually transmitted infection guideline and be checked against local resistance, because ceftriaxone and azithromycin susceptibility changes by region.

Commonest wrong answers: treating on the basis of a culture from the wrong site; missing a throat or rectal infection; forgetting that syphilis can mimic almost any single genital ulcer; treating the index patient without partner discussion; and failing to offer vaccination, screening and risk-reduction advice.

Full topics: infectious-disease/chlamydia, infectious-disease/gonorrhoea, infectious-disease/syphilis, infectious-disease/genital-ulcers-and-lesions, infectious-disease/vaginitis-and-vaginal-discharge, infectious-disease/hiv-prevention-prep-and-pep

Presenting picture: fever in a recently returned traveller, from same day of travel to months after return. Any traveller with malaria risk within the last 12 months must have malaria excluded first, whatever the prophylaxis history. Travel fevers fall into three time groups: under 2 weeks (dengue, chikungunya, influenza), 2 to 6 weeks (typhoid, hepatitis, malaria, acute HIV), and months to years (P. vivax and P. ovale malaria, tuberculosis, strongyloidiasis, schistosomiasis, leishmaniasis). A returned traveller with fever can look well one hour and critically ill the next, so admission should be easy. ^[7]

Single most likely diagnosis: malaria once an endemic area has been visited, followed by dengue in Latin America and Asia, and typhoid in South Asia. Most febrile travellers never have a tropical infection: pneumonia, urinary infection and diarrhoeal illness remain the bulk of the differential, and no diagnosis is ever reached in about a quarter of patients, most of whom recover anyway. Incubation, region and exposure narrow the list far more than the symptom names do. ^[7]

Best first test: a same-day malaria thick and thin film or rapid test, full blood count, biochemistry, blood cultures, arbovirus testing when indicated, and chest imaging where respiratory signs exist. If the Diamond Standard film is negative but suspicion is high, repeat it and re-test again. ^[6][7]

Best first treatment: any possibility of a life-threatening viral haemorrhagic fever demands immediate isolation, reporting and expert discussion. Otherwise start directed therapy based on the likely organism: effective antimalarials when malaria is confirmed or strongly suspected with severe illness, doxycycline when a rickettsial infection such as scrub or tick typhus is suspected, and appropriate antibiotics for typhoid guided by travel history and resistance patterns. Hospital-acquired or in-flight-acquired conventional infection should be covered where appropriate. Give empiric malaria treatment only when testing is delayed and pre-test probability is high. ^[6][7][11]

Commonest wrong answers: taking the travel history as a tick box instead of a narrative; forgetting malaria because the traveller took every tablet correctly; hunting for an exotic diagnosis while a routine urinary infection or pneumonia sits untreated; and delaying isolation until viral haemorrhagic fever is confirmed.

Full topic: infectious-disease/returning-traveller-with-fever

Presenting picture 1, bone infection: focal bone pain with warmth, swelling and often low-grade fever in an adult, and back pain that is worse at night in a person who has no relief from ordinary analgesia. A diabetic foot ulcer with exposed bone, a puncture wound, recent trauma, surgery or injecting drug use gives the route. Plain films can stay normal for 10 to 14 days, and vertebral change may take 2 to 8 weeks, so the clinical history often precedes the radiograph. ^[9]

Single most likely diagnosis: Staphylococcus aureus osteomyelitis, whatever the route in most settings. The rest of the picture comes from the route: Salmonella in sickle cell disease, Pseudomonas in puncture wounds and injecting drug use, Pasteurella after bites, mixed organisms in diabetic and traumatic wounds, and Mycobacterium tuberculosis in the spine.

Best first test: magnetic resonance imaging, which shows marrow change within days; two sets of blood cultures; C-reactive protein and erythrocyte sedimentation rate; bone biopsy (percutaneous or open) for histology and culture before antibiotics if the patient is stable. Superficial swab cultures do not predict the bone isolate and should not direct therapy. A raised erythrocyte sedimentation rate of 70 mm per hour or more in a person with a diabetic foot ulcer supports bone infection when combined with examination and simple imaging. ^[9][20]

Best first treatment: source control (debridement of dead bone, drainage of abscess, removal of infected hardware, revascularisation) combined with antibiotics. Chronic osteomyelitis in adults is treated with 4 to 6 weeks of parenteral beta-lactam therapy, then oral step-down with highly bioavailable agents once the patient is improving and cultures are known. Vertebral disease needs at least 6 weeks of antibiotics and urgent neurosurgical review for any new weakness, sensory loss or sphincter disturbance. Diabetic foot disease without surgical resection uses about 6 weeks, and up to 3 weeks after minor amputation with a positive bone margin. ^[9][21][20]

Commonest wrong answers: starting broad antibiotics before cultures in a stable patient; trusting a superficial swab; calling a degenerative Modic change on imaging an infection; and delaying referral when a spinal abscess may be forming.

Presenting picture 2, septic joint: an acutely hot, swollen and painful joint over a few hours, often the knee, usually with fever and severe pain on even the smallest movement. Every acutely hot joint is septic arthritis until proven otherwise; an acutely swollen single joint, however small, should not be labelled gout without aspiration. ^[5]

Single most likely diagnosis: native joint bacterial arthritis, most often staphylococcal, with Gram-negative organisms more likely in frail older or immunocompromised patients and in injecting drug users. Gout and pseudogout can coexist with it, so crystal examination does not exclude infection. ^[5]

Best first test: urgent sterile aspiration of the joint for white cell count, Gram stain, culture, and crystal examination, plus blood cultures and baseline inflammatory markers; ultrasound to localise difficult joints. ^[5]

Best first treatment: drainage of the joint, chosen with orthopaedics, combined with empiric antibiotics that always cover Staphylococcus aureus and add Gram-negative or MRSA cover on risk. Without drainage, cartilage damage can become irreversible over days. Oral step-down and duration follow the identified organism and local protocol. ^[22]

Commonest wrong answers: trusting a leucocyte count as the diagnosis; giving empiric antibiotics without washing out the joint; treating a crystal result as excluding infection; and missing a prosthetic joint infection that needs a separate, more demanding pathway. ^[3]

Full topics: infectious-disease/osteomyelitis, infectious-disease/septic-arthritis

Presenting picture: a subacute or chronic cough, haemoptysis, weight loss, fever and night sweats; or an asymptomatic person with a positive test of infection. Extrapulmonary disease can look like almost anything: tuberculous meningitis, miliary nodules, spinal kyphosis, a cold abscess, sterile pyuria, ascites or pericarditis. Remember that HIV, steroids at 15 mg prednisone equivalent daily or more, tumour necrosis factor blockers, diabetes and undernutrition convert latent infection into active disease, and tuberculosis can present with normal chest imaging in advanced HIV.

Single most likely diagnosis: active pulmonary tuberculosis in anyone with a persistent cough and risk factors, and latent tuberculosis infection in an asymptomatic contact with a positive tuberculin or interferon-gamma test. Latent infection is asymptomatic, non-infectious and needs active disease excluded before treatment, whereas active disease presents clinically and is usually bacteriologically confirmed when possible. ^[8]

Best first tests: sputum smear microscopy for acid-fast bacilli, nucleic acid amplification (Xpert MTB/RIF, which also reports rifampicin resistance), culture for speciation and drug susceptibility, and chest imaging. A tuberculin skin test of 5 mm or more is positive in high-risk or immunosuppressed people, 10 mm or more in other at-risk groups, and 15 mm or more without risk factors. An interferon-gamma release assay is preferred after BCG vaccination. Neither test separates latent infection from active disease, and both can be falsely negative in HIV and severe disease. ^[2][8][17]

Best first treatment: for drug-susceptible pulmonary tuberculosis, a 6-month regimen of 2 months of isoniazid, rifampicin, pyrazinamide and ethambutol (2HRZE), followed by 4 months of isoniazid and rifampicin (4HR). Treatment is directly observed. Latent infection is treated only after active disease is excluded and is best treated with short rifamycin-based regimens. People living with HIV should have antiretroviral therapy start within 2 weeks when the CD4 count is under 50 cells per microlitre and by 8 to 12 weeks otherwise, but not in the first 8 weeks with tuberculous meningitis. ^[8]

Commonest wrong answers: assuming BCG means lifelong protection; treating a positive tuberculin test as proof of active disease without excluding it; starting treatment and then not checking drug interactions with antiretrovirals; and missing that in untreated latent infection roughly four out of five tuberculosis cases in low-burden settings come from reactivation. ^[10]

Full topic: infectious-disease/tuberculosis

Presenting picture: measles, mumps and rubella, pertussis, diphtheria, tetanus, varicella, invasive pneumococcal, meningococcal or Haemophilus influenzae disease, hepatitis A or B, polio, rotavirus or human papillomavirus disease; in practice these present as fever with rash, paroxysmal cough with a whoop, flaccid paralysis or neonatal sepsis. Ask about age, vaccination status, pregnancy and immunosuppression before considering live vaccines, and remember that immunocompromised and unvaccinated patients suffer the severe forms. Hepatitis A remains one of the most common vaccine-preventable infections acquired during travel. ^[11]

Single most likely diagnosis: the organism differs by age and exposure. A febrile child with cough, coryza, conjunctivitis and Koplik spots is measles until proven otherwise; a baby with poor feeding, apnoea or a whoop is pertussis; a rash illness after travel may be varicella or an arbovirus; an unvaccinated adult with fever and rash may have measles. The key clinical discipline is to isolate before the result and ask about vaccination history before diagnosing.

Best first tests and treatment: many vaccine-preventable infections are diagnosed clinically with selective testing; for measles, send a serum sample for IgM and ask for respiratory PCR; for suspected pertussis, arrange nasopharyngeal PCR; for invasive disease, take blood and CSF cultures into the relevant bottles and start empiric antibiotics; for herpes simplex encephalitis, add aciclovir. Treatment is organism-directed and published in the disease topic: macrolides for pertussis and for contacts, specific antiviral therapy is not available for measles, and immunoglobulin given promptly can reduce severity in selected exposures. The vaccine history and isolation are at least as important as any test.

Commonest wrong answers: failing to isolate because "the disease is rare"; giving live vaccines in severe immunocompromise; assuming the family deliberately refused without asking; and treating measles as "just a rash" when it is a whole-body infection with lasting risk.

Full topics: infectious-disease/immunisation-and-travel-medicine, infectious-disease/measles-mumps-and-rubella, infectious-disease/diphtheria, infectious-disease/tetanus, infectious-disease/varicella-and-herpes-zoster, infectious-disease/poliomyelitis-and-acute-flaccid-paralysis

Presenting picture: a patient developing infection at least 48 hours after admission or within 30 days of surgery, with device-associated clues: fever with a new central line, dysuria with an indwelling catheter, new purulent wound, or ventilator-associated pneumonia after intubation. Remember that infection prevention is the treatment here: every suspected infection is also a possible failure of a routine process.

Single most likely diagnosis: healthcare-associated infection, and the common organisms are those under the device, on the skin or from the local environment: coagulase-negative staphylococci and Staphylococcus aureus with lines, Escherichia coli and other Enterobacterales with urinary catheters, and resistant organisms wherever antibiotics are overused. Surgical site, bloodstream and urinary tract are the common sites.

Best first tests: blood cultures from known lines and peripheral blood, urine culture through a newly inserted catheter, wound or device culture as appropriate, imaging for collections, and a review of whether the device is still needed. Clean hands before and after every patient contact, and whenever removing gloves, is part of every test and procedure rather than an optional extra. ^[10]

Best first treatment: treat the infection and remove the cause: taking out central lines, changing urinary catheters, draining collections, debriding wounds, and choosing antibiotics with the local team, with daily reassessment and de-escalation. Vasoactive sepsis, neutropenia and device-related bacteraemia are treated as sepsis. Prevention is better than treatment: a single room where possible, hand decontamination with handrub when hands are visibly clean and soap and water when hands are visibly soiled or when there is potential spread of alcohol-resistant organisms such as Clostridioides difficile, appropriate personal protective equipment, and safe injection practice, none of which are expensive. ^[10]

Commonest wrong answers: treating line culture positivity as contamination without clinical correlation; leaving a central line in place "because it is convenient"; prescribing perioperative antibiotics well outside the immediate perioperative window; and using alcohol handrub alone when hands are visibly contaminated. ^[10]

Full topic: infectious-disease/infection-prevention-and-control

• Fever of unknown origin remains largely infection, and tuberculosis must be excluded in every patient with prolonged fever. ^[1]
• Meningitis is treated before proof: blood cultures, lumbar puncture or imaging-driven antibiotics, ceftriaxone 2 g intravenously every 12 hours plus ampicillin 2 g intravenously every 4 hours for Listeria coverage, with aciclovir whenever herpes simplex encephalitis is possible. ^[3]
• Septic shock is an emergency, and the whole bundle matters: blood cultures, antibiotics within an hour, 30 mL per kilogram of crystalloid within 3 hours, noradrenaline rise to a MAP of 65 mm Hg, and early source control. ^[4]
• Advanced HIV disease is a CD4 count below 200 cells per microlitre; tuberculosis is the dominant co-infection, and antimicrobial therapy must match both diseases and their drug interactions. ^[5]
• Severe malaria needs intravenous artesunate 2.4 mg per kilogram at 0, 12 and 24 hours and a planned oral follow-on at 1 percent or less parasitaemia. ^[6]
• An acutely hot joint is septic arthritis until proven otherwise, and drainage plus antibiotics should not wait beyond the same day. ^[5]
• Tuberculosis asks for 2 months of HRZE then 4 months of HR, with directly observed therapy; latent infection is treated only after active disease is excluded and is about 90 percent effective. ^[8][10][16]
• Hospital infection is largely prevented by the basics: standard precautions, hand hygiene, aseptic technique, timely device removal and antimicrobial stewardship. ^[10]
• Infection spreads along geography, incubation and exposure. Take a full travel and contact history from every patient, and a second history when the first has not unlocked the diagnosis.

• fever-of-unknown-origin
• meningitis
• encephalitis
• sepsis
• hiv-infection
• malaria
• returning-traveller-with-fever
• chlamydia
• gonorrhoea
• syphilis
• osteomyelitis
• septic-arthritis
• tuberculosis
• immunisation-and-travel-medicine