Preparatory Mindset
Infectious diseases (ID) sits at the intersection of clinical medicine, microbiology, and epidemiology — it is the original "systems" specialty. Every clinical discipline eventually faces an ID question (fever of unknown origin, post-op wound infection, traveller returning from the tropics with diarrhoea). For exams (JNU finals, USMLE Step 1/2, LMCHK), ID rewards pattern recognition over rote: organism → typical exposure / risk → typical syndrome → first-line empiric therapy. Before opening a chapter on dengue or sepsis, always ask: *what is the likely pathogen category, what test confirms it, what is the first-hour intervention that saves life?*
The lecture series covers the Chinese statutory "法定传染病" mix: viral hepatitis, EHF, dengue, sepsis, brucellosis, AIDS, malaria, scrub typhus, cholera, and bacillary dysentery — weighted heavily toward vector-borne + zoonotic + enteric + blood-borne infections, with HIV as the chronic-immunodeficiency anchor.
Core Concepts
1. Definition and scope
Infectious disease = disease caused by pathogenic microorganisms (viruses, rickettsiae, bacteria, fungi, spirochetes) or parasites (protozoa, worms). Two sub-categories:
- Communicable disease (传染病) = infectious diseases that can spread under certain conditions and cause epidemics (cholera, plague, viral hepatitis, EHF, dengue, malaria, AIDS, dysentery).
- Non-communicable infection = infection that does not transmit person-to-person (e.g., liver abscess, carbuncle, most deep tissue infections).
> Quick check: *Liver abscess* and *carbuncle* are infections but not statutory communicable diseases. *Dengue, malaria, viral hepatitis, EHF* — communicable.
2. Three links of the infectious chain (传染过程三环节)
| Link | Definition | Block at |
|---|---|---|
| Source of infection (传染源) | Patient, carrier, or reservoir animal excreting the pathogen | Isolation, treatment, vector control |
| Route of transmission (传播途径) | How the pathogen reaches the next host | Hand hygiene, condoms, mosquito nets, safe water |
| Susceptible population (易感人群) | Non-immune persons who can be infected | Vaccination, prophylaxis |
Without all three, no transmission. Every prevention strategy attacks one link.
3. Infection vs. immunity (感染与免疫)
- Pathogen enters → replication → may cause disease (outcome depends on virulence × inoculum × host immunity).
- Innate immunity (barriers, phagocytes, NK cells, complement) holds the front line.
- Adaptive immunity (B-cell antibodies, T-cell cytotoxicity) clears infection and provides memory.
- Vaccines train adaptive immunity without disease — the cornerstone of prevention.
4. Basic clinical manifestations
Common patterns across the ID syllabus:
- Fever patterns — continuous (typhoid early), remittent (most viral), intermittent (malaria tertian/quartan), saddleback (dengue), biphasic (EHF).
- Rashes — maculopapular (measles, dengue), vesicular (chickenpox, hand-foot-mouth), petechial/purpuric (EHF, meningococcemia, dengue severe), eschar (scrub typhus).
- Hepatosplenomegaly + lymphadenopathy — viral hepatitis, EBV, HIV seroconversion, leishmaniasis.
- Diarrhoea syndromes — watery (cholera, ETEC), bloody/dysenteric (Shigella, EHEC, Entamoeba), profuse + rice-water stool (cholera).
5. Diagnosis, treatment, prevention
- Diagnosis: epidemiology + clinical syndrome + first-line specific tests (antigen, antibody, PCR, culture, blood film). Treat empirically when delay is dangerous (sepsis, cerebral malaria).
- Treatment: supportive care is often as important as specific antimicrobials (rehydration for cholera, organ support for sepsis, dialysis for EHF).
- Prevention: interrupt the chain; vaccine when available; vector control; report notifiable diseases within the statutory time window.
6. Current landscape (modern ID)
| Trend | Examples |
|---|---|
| Old enemies persist | Plague, cholera still endemic in some regions |
| Common infections remain prevalent | Viral hepatitis (esp. HBV in China), TB, influenza, infectious diarrhoea |
| Re-emergence of controlled disease | Gonorrhoea, syphilis (sexual transmission, behaviour change) |
| New pathogens | SARS, MERS, novel coronavirus (COVID-19), avian influenza (H5N1/H7N9), Ebola haemorrhagic fever, Legionella pneumonia, prion disease (mad-cow) |
> The COVID-19 pandemic proved that an "ID chapter" is never finished — pattern recognition and infection control principles learned in this course directly apply to the next emerging pathogen.
High-Yield Points
| Topic | Must-remember fact |
|---|---|
| Definition | Communicable disease requires transmission potential; liver abscess is infection but not communicable |
| Chain | Transmission needs source + route + susceptible host — break any one link |
| Fever patterns | Tertian = vivax/ovale malaria; Quartan = malariae; Saddleback = dengue; Biphasic = EHF |
| Vaccine-preventable (this syllabus) | HBV (HepB vaccine), HAV (HepA vaccine), cholera (oral), typhoid, JE |
| Notifiable timing (China) | Class A (plague, cholera) — 2 h; Class B (viral hepatitis, EHF, AIDS, malaria, dysentery) — 24 h |
| Empirical antimicrobial | Always pair to likely organism + local resistance; don't delay antibiotics in sepsis |
Topic Summary
Infectious diseases span the spectrum from common self-limiting viral illness to pandemic-class outbreaks. The ID toolkit is the same across diseases: identify the pathogen, recognise the syndrome, know the first-line diagnostic test, give the right antimicrobial early (when indicated), and prevent onward transmission. The 11 chapters ahead each apply this framework to a specific pathogen — viral hepatitis (chronic HBV/HCV), EHF (renal syndrome), dengue (saddleback fever + warning signs), sepsis (qSOFA + bundle), brucellosis (undulant fever + zoonosis), AIDS (CD4 staging), malaria (blood film + Anopheles), scrub typhus (eschar + Weil-Felix), cholera (rice-water stool + ORS), and bacillary dysentery (Shigella + bloody stool). The recurring theme is epidemiology + syndrome + first test + first antimicrobial.
LMCHK OSCE Practice — Fever in a Returning Traveller
Station setup: 35-year-old male, returned 10 days ago from rural Yunnan after a 3-week trek. Presents with daily fevers to 39–40 °C with rigors, headache, and myalgia. Two days ago he noticed dark urine and scleral icterus. Examination: T 39.2 °C, HR 110, BP 110/70, SpO₂ 98%. Mild scleral icterus, soft hepatomegaly 2 cm, no rash, no neck stiffness.
Candidate tasks (8 min):
- Take a focused travel/drug history (use mosquito nets? prophylaxis? sexual exposure? IVDU?).
- Propose a differential ranked by likelihood (malaria, dengue, viral hepatitis A/E, leptospirosis, typhoid, scrub typhus).
- State the single most important immediate test (thick + thin blood film for malaria parasites — falciparum is a medical emergency).
- Outline initial management if malaria is confirmed (IV artesunate, daily parasite count, supportive care).
Key marking cues:
- Recognises falciparum malaria = emergency; does not wait for travel history completeness before requesting blood film.
- Asks about prophylaxis (doxycycline / atovaquone-proguanil / mefloquine) and adherence.
- Differentiates hepatitis A/E (faecal-oral, incubation 2–6 wk) from HBV/HCV/HIV (blood/sexual).
- Asks about Leptospira exposure (river water, rat urine) — gives doxycycline empirically if suspected.
- Mentions notification as a statutory Class B disease once diagnosis confirmed.
Top differentials to commit to memory:
| Syndrome | Likely organism | First test |
|---|---|---|
| Fever + jaundice + travel | Malaria (P. falciparum), viral hepatitis A/E, leptospirosis | Blood film, LFT, IgM anti-HAV/HEV, Leptospira serology |
| Fever + rash + travel | Dengue, typhoid, measles, rickettsial | Dengue NS1/IgM, Widal, Weil-Felix |
| Fever + haemorrhage | EHF, dengue severe, meningococcemia | Platelet, Hantavirus IgM, dengue warning signs |