Infectious Diseases and Epidemiology: From Syndrome to Control
What the candidate must understand
Infectious disease questions combine clinical syndrome recognition with microbiology, antimicrobial choice, and infection control. Know the common community-acquired and healthcare-associated infections, the interpretation of cultures and biomarkers, and the principles of antimicrobial stewardship, including de-escalation and duration. Understand notifiable diseases, isolation precautions, and post-exposure prophylaxis. Epidemiology questions test incidence versus prevalence, sensitivity and specificity, positive and negative predictive values, outbreak investigation steps, and the difference between association and causation.
Traps that appear in questions
- Starting antibiotics for a viral syndrome without considering the harms of unnecessary treatment.
- Confusing colonization with infection when a culture is positive.
- Miscomputing predictive values by ignoring prevalence; sensitivity and specificity are prevalence-independent, predictive values are not.
- Forgetting standard precautions apply to all patients, with transmission-based precautions added for specific pathogens.
- Missing the need for post-exposure prophylaxis after a needlestick or a rabies exposure.
Worked example: predictive value
A test has 90% sensitivity and 95% specificity in a population with 1% prevalence. Among 10,000 people, 90 true positives and about 495 false positives occur, so the positive predictive value is roughly 15%. The rule of thumb: a positive result in a low-prevalence setting often needs confirmation, and prevalence must be stated before interpreting a test.
Rule of thumb
Match the antimicrobial to the likely pathogen and the site, then narrow based on culture and clinical response; document the indication and planned duration.