Communicable diseases

1. Biological basis and major disease-causing agents

Communicable diseases result from pathogens spreading directly or indirectly between hosts or from an environmental source. Infectious and contagious are not exact synonyms: contagious usually implies ready person-to-person spread. Tetanus is infectious but is not transmitted from person to person; infection follows contamination of wounds with spores of Clostridium tetani.

Bacteria are cellular organisms; tuberculosis is caused mainly by Mycobacterium tuberculosis, cholera by Vibrio cholerae and typhoid by Salmonella Typhi. Viruses require living host cells for replication. Measles, influenza, dengue, rabies, hepatitis B and COVID-19 are viral diseases. HIV attacks immune cells, especially CD4 T lymphocytes; untreated infection may progress to AIDS.

Protozoan diseases include malaria, caused by Plasmodium species, and visceral leishmaniasis or kala-azar, caused in India by Leishmania donovani. Helminths are parasitic worms responsible for conditions such as lymphatic filariasis and soil-transmitted helminth infections. Fungi cause infections such as ringworm, which is not a worm infestation.

Clinical severity depends on the organism, infectious dose, route of entry and host factors such as age, nutrition and immunity. Pathogenicity means the ability to cause disease; virulence refers to the degree of disease severity. These properties should not be confused with transmissibility.

• A carrier harbours a pathogen and may transmit it without showing obvious symptoms.
• An opportunistic infection takes advantage of weakened host defences; not every such infection spreads directly between people.

2. Transmission routes, reservoirs and vectors

The chain of infection comprises an infectious agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host. Breaking any link can prevent disease. Reservoirs include humans, animals, soil and water. Zoonoses are infections naturally transmissible between vertebrate animals and humans; rabies and Nipah virus infection are important examples.

Respiratory pathogens spread through infectious particles released during breathing, talking, coughing or sneezing. Larger particles tend to settle sooner, while smaller particles may remain suspended, especially indoors. Tuberculosis and measles are important airborne infections. Ventilation, appropriate masks and reduced crowding can lower exposure.

Faecal–oral transmission occurs when pathogens from faeces contaminate food, drinking water or hands. Cholera, hepatitis A, hepatitis E and poliomyelitis can spread through this route. Blood-borne and sexual transmission are important for HIV and hepatitis B; hepatitis C is predominantly blood-borne. Vertical transmission means transmission from mother to child during pregnancy, childbirth or breastfeeding, depending on the pathogen.

Vectors are living transmitters. Female Anopheles mosquitoes transmit malaria; female Aedes mosquitoes transmit dengue and chikungunya; sandflies transmit kala-azar. Aedes commonly breeds in water-holding containers and bites mainly during daylight. Anopheles generally bites between dusk and dawn. A mosquito is not itself the causative organism of these diseases.

• Biological vectors support pathogen development or multiplication; mechanical transmission involves passive carriage.
• Rabies usually follows exposure to saliva through bites or scratches from infected mammals.

3. Epidemiology, detection and outbreak control

Endemic describes the usual presence of a disease within a population or geographical area. An epidemic is occurrence above the expected level; outbreak often denotes a more localised event. A pandemic is an epidemic extending across countries or continents with widespread transmission. The term does not, by itself, indicate clinical severity.

Incidence measures new cases arising in a population during a period, whereas prevalence measures existing cases at a point or over a period. The incubation period runs from infection to symptom onset. It differs from the latent period, which runs from infection to becoming infectious. Consequently, transmission may begin before symptoms appear.

The basic reproduction number, R0, is the expected number of secondary infections generated by one infected person in a wholly susceptible population under specified conditions. The effective reproduction number reflects current immunity and interventions. Sustained transmission generally expands when it exceeds one. Neither value is an unchanging property of a pathogen alone.

Diagnostic methods include microscopy, culture, antigen detection, antibody tests and nucleic acid amplification tests. PCR detects genetic material; RT-PCR first converts RNA into DNA. Antibodies may indicate previous exposure or vaccination and do not always establish current infection. Test interpretation depends on timing, sensitivity, specificity and disease prevalence.

• Isolation separates infected people; quarantine restricts exposed people who may become infectious.
• Outbreak investigation combines case definitions, laboratory confirmation, contact tracing and analysis by person, place and time.

4. Prevention, immunity and treatment

Primary prevention acts before disease develops through vaccination, safe water, sanitation, hygiene, vector control and safer sexual practices. Secondary prevention includes early detection and prompt treatment. Tertiary prevention reduces complications and disability. Source reduction, insecticide-treated nets and indoor residual spraying have different roles depending on vector behaviour.

Active immunity develops after infection or vaccination and generally involves immune memory. Passive immunity transfers ready-made antibodies, as through maternal antibodies or immunoglobulin administration, giving immediate but usually temporary protection. Vaccines may use live attenuated organisms, inactivated pathogens, protein components, toxoids, viral vectors or nucleic acids.

Herd protection occurs when sufficient population immunity reduces transmission and indirectly protects susceptible people. Its extent depends on vaccine effectiveness against infection and transmission, coverage and population mixing. It is not applicable in the same way to tetanus because tetanus does not spread between people. Vaccination does not necessarily produce complete protection against infection.

Treatment is pathogen-specific: antibiotics treat susceptible bacterial infections, antimalarials treat malaria and antiretroviral therapy suppresses HIV replication. Misuse of antimicrobials and inadequate infection control select and spread resistant organisms. Multidrug-resistant tuberculosis is resistant to at least isoniazid and rifampicin. Oral rehydration solution replaces water and electrolytes lost during diarrhoea; it does not directly kill the pathogen.

• Suspected rabies exposure requires immediate thorough wound washing and prompt assessment for vaccination and, when indicated, rabies immunoglobulin.

5. India’s public-health response and emerging challenges

The Integrated Disease Surveillance Programme, coordinated through the National Centre for Disease Control, supports surveillance and outbreak response. The Integrated Health Information Platform enables digital reporting. The Indian Council of Medical Research contributes research and laboratory expertise, while the National Centre for Vector Borne Diseases Control supports programmes addressing major vector-borne diseases.

The Universal Immunisation Programme provides vaccines against designated vaccine-preventable diseases. Mission Indradhanush, launched in 2014, seeks to improve coverage among unvaccinated and partially vaccinated children and pregnant women. The National Tuberculosis Elimination Programme uses diagnosis, treatment, notification and adherence support; Ni-kshay is its digital platform.

Control reduces disease burden to an acceptable level. Elimination of transmission means zero incidence of infection in a defined geographical area, requiring continued preventive measures. Elimination as a public-health problem instead means meeting specified thresholds, not necessarily zero transmission. Eradication means permanent worldwide reduction to zero incidence of infection through deliberate efforts.

Emerging risks include antimicrobial resistance, rapid travel, urban crowding, land-use change and climate-sensitive vector distribution. One Health links human, animal and environmental health through coordinated surveillance and action. India’s Epidemic Diseases Act, 1897 provides powers for epidemic response; the International Health Regulations, 2005 establish international obligations for detecting, assessing and reporting relevant public-health events.

• India’s stated tuberculosis elimination target was 2025; a policy target must not be confused with verified achievement.

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