Respiratory Infections

Community-Acquired Pneumonia (CAP)

•    Most common: Streptococcus pneumoniae

•    Other organisms:

o    Mycoplasma pneumoniae ("atypical"; young adults, often mild, extra-pulmonary features like haemolysis)

o    Legionella pneumophila (hyponatraemia, confusion, diarrhoea; linked to water systems)

o    Haemophilus influenzae (esp. in COPD)

o    Staphylococcus aureus (post-influenza pneumonia)

•    Severity assessment: CURB-65 score (Confusion, Urea >7, RR ≥30, BP low, age ≥65)

Hospital-Acquired Pneumonia (HAP)

•    Occurs ≥48 hours after admission

•    Common organisms:

o    Gram-negative bacilli: Pseudomonas, E. coli, Klebsiella

o    Staphylococcus aureus, including MRSA

Tuberculosis (TB)

•    Reactivation in upper lobes ("Simon focus"), cavitation

•    Histology: caseating granulomas with Langhans giant cells

•    Extra-pulmonary: lymph nodes, CNS (tuberculoma, meningitis), bones (Pott's disease)

Pneumocystis jirovecii Pneumonia (PCP)

•    Seen in: HIV (CD4 <200), steroids

•    Features: dry cough, exertional dyspnoea, hypoxia disproportionate to CXR

•    CXR: bilateral "ground-glass" infiltrates

•    Lab: ↑ LDH, β-D-glucan positive

•    Rx: high-dose co-trimoxazole ± steroids (if severe hypoxia)

Neurological Infections

Meningitis

Bacterial causes

•    S. pneumoniae: most common in adults

•    N. meningitidis: rapid progression; characteristic purpuric/petechial rash

•    Listeria monocytogenes: elderly, immunosuppressed, pregnant; add ampicillin to empiric therapy

Viral causes

•    Enteroviruses (coxsackie, echovirus): most common viral cause

•    HSV-2: can cause recurrent aseptic meningitis (Mollaret)

Encephalitis

•    HSV-1: most common cause of sporadic encephalitis

o    Temporal lobe involvement → personality change, aphasia, olfactory hallucinations

•    Diagnosis: MRI (temporal hyperintensity), CSF PCR for HSV

•    Rx: IV aciclovir

Brain abscess

•    Organisms: streptococci (esp. viridans group), anaerobes; post-sinusitis or otitis

•    Imaging: ring-enhancing lesion on MRI/CT

•    Rx: antibiotics (e.g., ceftriaxone + metronidazole) ± surgical drainage

Gastrointestinal Infections

Bacterial causes

•    Campylobacter jejuni: most common bacterial gastroenteritis in UK; associated with Guillain–Barré syndrome

•    Salmonella enterica: often from poultry; causes typhoidal or non-typhoidal disease

•    Shigella: bloody diarrhoea; risk of HUS in children

•    E. coli O157:H7: shiga toxin → bloody diarrhoea, risk of haemolytic uraemic syndrome (HUS)

Viral causes

•    Norovirus: cruise ships, care homes; profuse vomiting

•    Rotavirus: most common cause in children worldwide

Parasitic causes

•    Giardia lamblia: foul-smelling, greasy stools; from contaminated water

•    Entamoeba histolytica:

o    Dysentery (bloody diarrhoea)

o    Liver abscess ("anchovy paste" pus); right lobe common

C. difficile infection

•    Usually post-antibiotics (e.g., clindamycin, cephalosporins)

•    Features: diarrhoea, pseudomembranous colitis

•    Rx:

o    Mild/moderate: oral vancomycin (first-line)

o    Severe: oral vancomycin ± IV metronidazole

o    Fulminant: may require colectomy

Soft Tissue and Bone Infections

Cellulitis

•    Organisms: Group A streptococcus (GAS), Staphylococcus aureus

•    Features: erythema, warmth, tender swelling, clear demarcation

Necrotising fasciitis

•    Organisms: GAS, mixed anaerobes

•    Severe pain out of proportion to skin changes

•    Surgical emergency: early debridement + broad-spectrum antibiotics

Diabetic foot infections

•    Polymicrobial: Gram-positives, Gram-negatives, anaerobes

•    Often associated with osteomyelitis

Osteomyelitis

•    S. aureus: most common overall

•    Salmonella: classic in sickle cell disease

•    Diagnosis: MRI most sensitive early; bone biopsy definitive

Septic arthritis

•    S. aureus: most common

•    Neisseria gonorrhoeae: sexually active young adults

•    Commonly monoarticular (knee most common)

•    Diagnosis: joint aspiration (WBC >50,000, purulent fluid)

•    Rx: urgent joint washout + IV antibiotics

Extra Revision Pearls

•    Meningococcal prophylaxis: close contacts get rifampicin or ciprofloxacin

•    Legionella clues: hyponatraemia, deranged LFTs, recent hotel stay

•    E. coli O157: avoid antibiotics → may worsen HUS risk

•    S. aureus: also common in IV drug users (right-sided endocarditis)