Infections

A dysregulated host response to infection causing life-threatening organ dysfunction. Sepsis = infection + organ dysfunction; septic shock = sepsis with persistent hypotension needing vasopressors and a raised lactate despite fluid resuscitation. The whole game is EARLY recognition (NEWS2/red-flag criteria, qSOFA) and the SEPSIS SIX within the first hour — antibiotics must not wait for a confirmed source. (The full resuscitation algorithm is on acute_care; this is the infection-recognition hub.)

Classically a fever >38.3°C on several occasions for more than 3 weeks that remains undiagnosed after appropriate initial investigation. The value of PUO is the structured differential: INFECTION (abscess, TB, endocarditis, HIV), MALIGNANCY (lymphoma, leukaemia, renal cell carcinoma), CONNECTIVE-TISSUE/autoimmune disease (adult Still's, vasculitis, GCA/PMR), and MISCELLANEOUS (drugs, thromboembolism, factitious). The approach is methodical, history-driven, and avoids blind antibiotics.

A fulminant toxin-mediated illness in which bacterial superantigens (staphylococcal TSST-1, or group A streptococcal exotoxins) trigger massive cytokine release, causing high fever, a diffuse erythematous rash, hypotension and rapid multi-organ failure. Classically linked to tampon use or wound/skin infection (staphylococcal) or to invasive group A strep (often with necrotising fasciitis). It is an emergency: remove the source, resuscitate, and give antibiotics ± IVIG.

Infections acquired as a consequence of healthcare, conventionally arising >48 hours after admission or related to a device/procedure. The high-yield set: Clostridioides difficile (antibiotic-associated diarrhoea), MRSA, central-line-associated bloodstream infection, catheter-associated UTI, ventilator-associated pneumonia, and surgical site infection. Prevention (hand hygiene, antimicrobial stewardship, device care, bundles) and prompt source control are the recurring themes.

A localised collection of pus walled off from surrounding tissue, most often staphylococcal. It can be superficial (skin/soft tissue) or deep (intra-abdominal, hepatic, perianal, psoas, dental, brain). The cardinal principle that the exam tests relentlessly: PUS MUST BE DRAINED — antibiotics alone do not clear an undrained abscess. Deep abscesses present as a focus of sepsis or PUO and need imaging to find and drain them.

An acute seasonal respiratory infection caused by influenza A or B, spread by droplets. Most cases are self-limiting, but it causes significant morbidity in at-risk groups (elderly, pregnant, chronic disease, immunosuppressed) — primarily through complications, especially secondary bacterial pneumonia (classically Staph. aureus). Management is supportive for the well; neuraminidase inhibitors (oseltamivir) are for at-risk patients within 48 hours, and annual vaccination is the key prevention.

Infection with SARS-CoV-2, ranging from asymptomatic/mild upper-respiratory illness to viral pneumonia, ARDS and a hyperinflammatory/thrombotic syndrome. Risk of severe disease rises with age, obesity, immunosuppression and chronic disease. Management is tiered by severity: supportive for mild disease; antivirals for high-risk non-hospitalised patients; and dexamethasone (± immunomodulators) plus thromboprophylaxis for those needing oxygen.

A clinical syndrome ('glandular fever'), usually from Epstein-Barr virus, of fever, exudative pharyngitis and lymphadenopathy in adolescents/young adults, with atypical lymphocytes and a positive heterophile-antibody (Monospot/Paul-Bunnell) test. CMV and toxoplasmosis cause a Monospot-negative mononucleosis-like illness. Two high-yield traps: a maculopapular rash if amoxicillin/ampicillin is given, and splenic rupture risk — so avoid contact sports.

Three vaccine-preventable, NOTIFIABLE viral exanthems, increasingly seen where MMR uptake has fallen. Measles (prodrome + Koplik spots + descending rash, the most dangerous — pneumonia/encephalitis/SSPE), mumps (parotitis ± orchitis/meningitis), and rubella (mild rash + lymphadenopathy, but devastating CONGENITAL rubella syndrome in early pregnancy). Management is supportive; the priorities are notification, isolation, and protecting susceptible contacts (especially pregnant women). Paediatric detail is on child_health.

Primary VZV infection is chickenpox (varicella) — a vesicular rash in crops; the virus then lies latent in dorsal-root ganglia and can reactivate as shingles (herpes zoster) — a painful dermatomal vesicular rash. The high-yield concerns are chickenpox in pregnancy/the immunocompromised/neonates, the contagious period, ophthalmic and Ramsay Hunt zoster, and post-herpetic neuralgia. Aciclovir is used for at-risk/zoster; VZIG protects vulnerable exposed contacts.

A retrovirus that progressively destroys CD4 T-lymphocytes, causing immunodeficiency. It is the hub of the immunocompromised host: the CD4 count predicts which opportunistic infections and malignancies occur. Presentations range from a seroconversion illness, through an asymptomatic latent phase, to AIDS-defining opportunistic disease. Antiretroviral therapy (ART) is for everyone, restores immunity, and renders the virus untransmittable when suppressed (U=U). (Transmission/PrEP/PEP detail is on sexual_health.)

A protozoan infection transmitted by the Anopheles mosquito, and the most important diagnosis to consider in any febrile returning traveller. Plasmodium falciparum causes the severe, potentially fatal disease; vivax and ovale form dormant liver hypnozoites that cause relapse. Diagnosis is by thick and thin blood films (×3). Severe falciparum malaria is a medical emergency treated with IV artesunate; uncomplicated falciparum with an oral artemisinin-combination therapy (ACT).

A systemic infection by Salmonella Typhi/Paratyphi, acquired faecal-orally (contaminated food/water), classically in travellers returning from South Asia. Unlike non-typhoidal Salmonella gastroenteritis, it presents as a systemic febrile illness with relative bradycardia, abdominal symptoms and (later) complications such as GI bleeding/perforation. Blood cultures diagnose it; treatment is increasingly with ceftriaxone or azithromycin because of fluoroquinolone resistance.

Infection with the protozoan Toxoplasma gondii, acquired from cat faeces or undercooked meat. In the immunocompetent it is usually asymptomatic or a self-limiting mononucleosis-like illness. The two settings that matter: the immunocompromised (especially HIV with CD4 <100 → cerebral toxoplasmosis with ring-enhancing lesions) and congenital infection (primary maternal infection in pregnancy → the classic triad).

Infections transmitted from animals or their vectors, where the EXPOSURE HISTORY is the diagnostic key. Lyme disease (Borrelia burgdorferi, from Ixodes tick bites) is the headline — erythema migrans is diagnostic and treated with doxycycline. Other examinable zoonoses include leptospirosis (fresh water/rats), brucellosis (unpasteurised dairy/livestock), Q fever (farm animals), rabies (animal bite) and cat-scratch disease — each pointed to by occupational, recreational or travel exposure.

A potentially fatal disease caused by the neurotoxin (tetanospasmin) of Clostridium tetani, which enters through a contaminated (especially deep/dirty/devitalised) wound and blocks inhibitory neurotransmission, causing severe muscle spasm and autonomic instability. It is now rare in vaccinated populations, so the high-yield exam content is wound-based prophylaxis (by wound type and immunisation status) rather than treatment of established disease.

Parasitic worm infections, grouped into nematodes (roundworms), cestodes (tapeworms) and trematodes (flukes). Most are travel- or soil/food-related and many are mild, but several are high-yield: threadworm (the common UK childhood worm), schistosomiasis (a key travel diagnosis linked to bladder cancer), strongyloides (life-threatening hyperinfection in the immunosuppressed), and tapeworm cysts (hydatid disease, neurocysticercosis). Eosinophilia in a traveller is a classic clue. Most respond to a '-bendazole' or praziquantel.

The commonest cause of skin/soft-tissue infection, abscess, and — crucially for the exam — the pathogen to reach for in several classic contexts. Coagulase-POSITIVE (vs coag-negative S. epidermidis). Nasal carriage in ~30%. MRSA (methicillin-resistant) needs a glycopeptide (vancomycin/teicoplanin) — β-lactams (flucloxacillin, co-amoxiclav) DO NOT cover it. Toxin-mediated disease: toxic shock syndrome (TSST-1), scalded skin, food poisoning (rapid, preformed enterotoxin).

Streptococci are Gram-positive cocci in CHAINS (catalase-negative — distinguishing them from staph). The exam tests species-by-context: **S. pyogenes** (Group A, β-haemolytic) — tonsillitis, scarlet fever, erysipelas/impetigo, necrotising fasciitis, and post-infectious rheumatic fever & glomerulonephritis; **S. pneumoniae** (α-haemolytic, diplococci) — commonest CAP, otitis media, bacterial meningitis in adults; **S. viridans** — subacute endocarditis on abnormal valves after dental work; **S. bovis/gallolyticus** — endocarditis associated with COLORECTAL CANCER (investigate the bowel); **S. agalactiae** (Group B) — neonatal sepsis/meningitis.

A grouping of the high-yield Gram-negatives and anaerobes tested by context. **E. coli** — commonest cause of UTI and Gram-negative sepsis; O157:H7 (Shiga toxin) → HUS (avoid antibiotics). **Pseudomonas aeruginosa** — burns, hot-tub folliculitis, otitis externa, cystic fibrosis, blue-green pus; needs anti-pseudomonal cover (pip-tazobactam, ceftazidime, ciprofloxacin). **Klebsiella** — cavitating pneumonia in alcoholics, UTI. **Neisseria meningitidis** (Gram-negative diplococcus) — meningococcal sepsis. Gram-negative sepsis is driven by **lipopolysaccharide (LPS) endotoxin**. Anaerobes (Bacteroides, Clostridia) → intra-abdominal/aspiration infection, gas gangrene — covered by metronidazole.

Infection of a heart valve/endocardium, diagnosed on the modified Duke criteria (blood cultures + echo vegetation). The exam pivots on **organism-by-context**: **S. aureus** (commonest overall, acute, IV drug users → tricuspid/right-sided); **S. viridans** (subacute, abnormal native valve after dental work); **S. bovis/gallolyticus** (→ colorectal cancer); **coagulase-negative staph (S. epidermidis)** (early prosthetic-valve, <1 yr); **Enterococcus** (after GI/GU procedures); **HACEK** & Coxiella/Bartonella (culture-negative). Stigmata: splinter haemorrhages, Osler's nodes, Janeway lesions, Roth spots, new murmur, emboli.

Acute diarrhoea ± vomiting, usually self-limiting and managed with rehydration. The exam tests **organism-by-context**: **Norovirus/rotavirus** (commonest, watery, outbreaks/children); **Campylobacter** (commonest bacterial — undercooked poultry, bloody, may mimic appendicitis; a/w Guillain–Barré, reactive arthritis); **Salmonella/Shigella** (bloody, food/travel); **E. coli O157** (bloody → HUS, no antibiotics); **C. difficile** (post-antibiotic, esp. co-amoxiclav/clindamycin/cephalosporins/PPI); **Cryptosporidium** (watery, immunocompromised/HIV, modified Ziehl–Neelsen); **Giardia** (prolonged, bloating/malabsorption, travel/hikers); **Staph aureus/Bacillus cereus** (rapid, preformed toxin — reheated rice for B. cereus). Most need only oral rehydration; antibiotics are the exception.

Infection of the meninges — a medical emergency. **Organism by age/host**: neonates → Group B strep, E. coli, Listeria; children/adults → Neisseria meningitidis (meningococcus) & Streptococcus pneumoniae (pneumococcus); elderly/immunocompromised/pregnant → add Listeria (needs amoxicillin cover). Meningococcal disease adds a **non-blanching purpuric rash** (meningococcaemia). Classic triad: fever, headache, neck stiffness (± photophobia, Kernig/Brudzinski). Give antibiotics IMMEDIATELY — do not delay for imaging/LP. CSF: bacterial = ↑neutrophils, ↑protein, ↓glucose, turbid.