Communicable Diseases - A Detailed Overview

Definition

"An illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment."

• Park's Textbook of Preventive and Social Medicine

The Chain of Infection (Three Links)

Link 1: Source / Reservoir of Infection

• Most communicable diseases are contracted from human sources.
• Includes both cases (symptomatic individuals) and carriers (those who harbour the agent without showing symptoms).
• Carrier types include:
  • Incubatory carrier - infectious during incubation (e.g., measles)
  • Convalescent carrier - still infectious after recovery (e.g., typhoid)
  • Healthy carrier - never develops disease but spreads it (e.g., cholera, diphtheria)
  • Chronic carrier - carries the organism for months/years (e.g., hepatitis B)

• Diseases transmitted from animals to humans are called zoonoses.
• Examples: rabies (dogs), plague (rodents), brucellosis (cattle/goats), anthrax, leptospirosis, kyasanur forest disease.
• Sub-types:
  • Anthropozoonoses: transmitted from animals to man (rabies, anthrax)
  • Zooanthroponoses: from man to animals (human TB in cattle)
  • Amphixenoses: maintained in both (e.g., S. japonicum)

• Soil (tetanus, mycosis, hookworm larvae), water (cholera, typhoid), and inanimate objects act as reservoirs or vehicles of infection.

Link 2: Modes of Transmission

A. Direct Transmission

B. Indirect Transmission (the 5 Fs)

1. Vehicle-borne (Food, Water, Milk): The infectious agent is carried by contaminated food, water, milk, or biological products. Examples: cholera, typhoid, hepatitis A, polio (faecal-oral route).
2. Vector-borne: Transmitted via living organisms (arthropods):
   • Mechanical - simple carriage (flies spreading typhoid)
   • Biological - agent multiplies/develops inside vector (mosquito → malaria, dengue; ticks → Lyme disease; sandfly → kala-azar)
3. Airborne (Droplet nuclei): Residue of evaporated droplets (<5 µm) that remain suspended in air for long periods and travel far. Examples: TB, measles, chickenpox, Q fever.
4. Fomite-borne: Transmission via contaminated inanimate objects (soiled linen, clothes, handkerchiefs, surgical instruments). Examples: conjunctivitis, ringworm, smallpox.
5. Unclean hands/fingers: A common route for many gastrointestinal and respiratory infections.

Link 3: Susceptible Host

• Age (extremes of age are more vulnerable)
• Nutritional status (malnutrition impairs immunity)
• Immune status (prior infection, vaccination, passive immunity)
• Genetic factors (sickle cell trait offers partial malaria protection)
• Comorbidities (diabetes, HIV, malignancy)

Key Epidemiological Terms

Classification of Communicable Diseases by Type

Important Communicable Disease Problems in India

• Malaria - Major problem; P. falciparum proportion rising
• Tuberculosis - India accounts for ~1/5 of global TB incidence; HIV-TB co-infection and MDR-TB are major concerns
• Diarrhoeal diseases - Leading cause of morbidity/mortality in children under 5
• Acute Respiratory Infections (ARI) - Major killer in children under 5
• HIV/AIDS - Estimated 2.1 million HIV-positive cases (2017 data)
• Leprosy, Filariasis, Kala-azar, Dengue, Viral Hepatitis, Japanese Encephalitis - All remain significant public health challenges

Prevention and Control

1. Control at Source/Reservoir

• Early diagnosis and treatment - renders cases non-infectious
• Isolation of infectious cases
• Notification and contact tracing
• Treatment of carriers (e.g., typhoid carrier gets antibiotics)
• Vector control (insecticides, larval control, bed nets for malaria)
• Animal source control (rabies vaccination of dogs, pasteurisation of milk, slaughter of infected animals)

2. Breaking Modes of Transmission

• Safe water supply and food safety (boiling water, pasteurisation, sanitary food handling)
• Environmental sanitation - proper sewage disposal, toilet facilities (at least 1 sanitary convenience per 25 employees), garbage disposal
• Hand hygiene and personal hygiene practices
• Use of PPE, masks (especially for airborne diseases)
• Transmission-based precautions in hospitals: contact precautions, droplet precautions, and airborne precautions (Harrison's Principles of Internal Medicine)
• Sterilisation and disinfection of fomites and medical equipment
• Safe sex practices, needle exchange programmes

3. Protection of the Susceptible Host

• Immunisation (active and passive) - the most powerful tool; vaccines for TB (BCG), polio, measles, hepatitis B, typhoid, yellow fever, influenza, COVID-19
• Chemoprophylaxis - e.g., antimalarials for travellers, cotrimoxazole in HIV patients
• Improved nutrition - strengthens immune responses
• Health education - promoting hygiene, vaccination uptake, safe behaviours
• Screening programmes for early detection

4. Surveillance and Outbreak Response

Key Measures at a Glance

COMMUNICABLE DISEASE CONTROL TRIANGLE
           AGENT
          /     \
        HOST --- ENVIRONMENT

Interrupt any corner of this triangle
= break the chain = control the disease

• Park's Textbook of Preventive and Social Medicine (primary reference for all definitions, chain of infection, transmission modes, and India-specific data)
• Harrison's Principles of Internal Medicine, 22nd Ed. (2025) - transmission-based precautions in healthcare settings
• WHO on Communicable Diseases - global burden and SDG targets