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Part of the FMGE study roadmap. Botany topic psm-002 of Botany.

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Communicable Diseases and Epidemiological Determinants

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Communicable Diseases and Epidemiological Determinants — Key Facts for FMGE Core concept: Understanding the chain of infection and modes of transmission is essential for disease prevention and control High-yield point: The “host-agent-environment” model (epidemiological triad) explains how diseases spread; breaking any link prevents transmission ⚡ Exam tip: Know the incubation periods and characteristic presentations of major communicable diseases and their prevention strategies

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Communicable Diseases and Epidemiological Determinants — FMGE Study Guide

Chain of Infection

Steps in Chain

  1. Pathogen (Infectious Agent): Bacteria, virus, fungus, parasite
  2. Reservoir: Where the agent lives and multiplies (humans, animals, environment)
  3. Portal of Exit: How the agent leaves the reservoir (respiratory, GI, blood, skin)
  4. Mode of Transmission: How the agent reaches the host
  5. Portal of Entry: How the agent enters the new host
  6. Susceptible Host: Person who can acquire the infection

Breaking the Chain

  • Eliminate reservoir: Treatment, isolation, culling animals
  • Block exit: Hand hygiene, masks, wound care
  • Prevent transmission: Quarantine, disinfection, vector control
  • Block entry: PPE, vaccination, prophylaxis
  • Protect host: Vaccination, healthy lifestyle, chemoprophylaxis

Modes of Transmission

Direct Transmission

  • Direct contact: Touching, biting, sexual contact
  • Droplet spread: Large droplets (>5 μm) travel <1 meter (coughing, sneezing, talking)
    • Examples: Influenza, meningitis, pertussis
  • Vertical transmission: Mother to child (transplacental, during birth, breastfeeding)

Indirect Transmission

  • Vehicle-borne: Contaminated food, water, blood, objects (fomites)
  • Vector-borne: Insects (mosquitoes, flies, ticks)
    • Mechanical: On legs/body surface
    • Biological: Multiplication in vector (malaria in Anopheles)
  • Airborne: Small droplets (<5 μm) remain suspended and travel >1 meter
    • Examples: Measles, varicella, TB, COVID-19
  • Droplet nuclei: Evaporated droplets that become small enough to be inhaled

Herd Immunity

  • When enough people in a population are immune, chain of transmission breaks
  • Herd immunity threshold: Varies by disease (measles ~95%, polio ~80%)
  • Achieved through vaccination or natural infection

Classification of Diseases

Endemic

  • Constant presence of a disease in a geographic area at a consistent rate
  • Example: Malaria in certain tropical regions

Epidemic

  • Increase in cases above expected baseline in a defined area/population
  • Example: COVID outbreak in Wuhan, influenza outbreaks

Pandemic

  • Global spread of a new disease across countries/continents
  • Examples: COVID-19, H1N1 influenza 2009, HIV/AIDS

Outbreak

  • Similar to epidemic but often more localized or contained

Zoonosis

  • Diseases transmitted from animals to humans
  • Examples: Rabies, brucellosis, plague, avian influenza

Nosocomial Infections

  • Infections acquired in hospital (hospital-acquired/associated)
  • Common pathogens: E. coli, Staphylococcus aureus, Pseudomonas
  • Surgical site infections, UTIs (most common), pneumonia, bloodstream infections

Specific Communicable Diseases

Tuberculosis

  • Causative agent: Mycobacterium tuberculosis (acid-fast bacillus)
  • Mode of transmission: Airborne (droplet nuclei)
  • Incubation period: Weeks to years (initial infection → primary TB)
  • Types: Primary TB (lower lobe, Ghon complex), Secondary/reactivation TB (apical cavitary)
  • Diagnosis: AFB smear, sputum culture, GeneXpert, Mantoux test (PPD)
  • BCG vaccine: Live attenuated M. bovis; not fully protective
  • DOTS: Directly observed treatment, short-course - WHO strategy

HIV/AIDS

  • Mode of transmission: Sexual, blood, vertical (mother to child)
  • Window period: 2 weeks to 3 months (antibodies not yet detectable)
  • Diagnosis: ELISA (screening), Western blot (confirmatory), rapid tests
  • Prevention: ABC (Abstinence, Be faithful, Condoms), PMTCT, blood safety, harm reduction
  • ART (Antiretroviral therapy): Reduces viral load, prevents transmission, improves survival
  • Opportunistic infections: PCP (pneumocystis jirovecii pneumonia), oral candidiasis, toxoplasmosis, cryptococcal meningitis, TB

Viral Hepatitis

  • Hepatitis A: Fecal-oral; acute; no chronic carrier state; fecal-oral
  • Hepatitis B: Blood, sexual, vertical; chronic carrier state; HBV vaccine available
  • Hepatitis C: Blood; chronic carrier state; no vaccine
  • Hepatitis E: Fecal-oral; especially dangerous in pregnancy

Cholera

  • Causative agent: Vibrio cholerae (comma-shaped bacterium)
  • Mode: Water contamination; fecal-oral
  • Presentation: Rice-water stools, severe dehydration
  • Treatment: ORS, antibiotics (azithromycin, doxycycline)
  • Prevention: Water treatment, sanitation, ORS preparation education

Dengue

  • Vector: Aedes aegypti mosquito
  • Presentation: High fever, headache, retro-orbital pain, rash, thrombocytopenia
  • Dengue hemorrhagic fever: Plasma leakage, bleeding, shock
  • Prevention: Mosquito control, personal protection (repellents, nets)

Malaria

  • Vector: Anopheles mosquito
  • Species: P. falciparum (most dangerous), P. vivax, P. ovale, P. malariae, P. knowlesi
  • Presentation: Cyclic fevers (tertian/quartan depending on species)
  • Treatment: ACT (artemisinin-based combination therapy) for P. falciparum
  • Prevention: ITNs (insecticide-treated nets), IRS (indoor residual spraying), chemoprophylaxis

Typhoid

  • Causative agent: Salmonella typhi
  • Mode: Fecal-oral; chronic carriers (Mrs. Mallon/Typhoid Mary)
  • Presentation: Step-ladder fever, rose spots, hepatosplenomegaly
  • Diagnosis: Widal test, blood/stool culture
  • Vaccine: Ty21a (oral), Vi polysaccharide (injectable)

Immunization

Types of Immunity

  • Active: Body produces antibodies; long-lasting; takes time to develop
  • Passive: Preformed antibodies given; immediate but short-lasting

Vaccine Types

  • Live attenuated: MMR, Varicella, Rotavirus, BCG, Yellow fever, Oral polio
  • Inactivated/Killed: Hepatitis A, Influenza (injection), Rabies, IPV
  • Toxoid: Tetanus, Diphtheria (inactivated toxin)
  • Polysaccharide/Conjugate: Hib, Pneumococcal, Meningococcal (T-cell independent - less durable)
  • mRNA/Viral vector: COVID-19 vaccines

National Immunization Schedule (India)

  • BCG: At birth
  • Hep B: At birth
  • OPV: At birth, 6, 10, 14 weeks
  • DPT: 6, 10, 14 weeks
  • Hib: 6, 10, 14 weeks
  • Rotavirus: 6, 10, 14 weeks
  • IPV: 14 weeks
  • MR: 9 months
  • Vitamin A: 9 months
  • Japanese Encephalitis: In endemic areas
  • DPT booster: 16-24 months, 5-6 years

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