Community medicine chapters
Infectious disease epidemiology
The chain of infection from reservoir to susceptible host, types of carriers, direct and indirect modes of transmission, incubation, latent and communicable periods, and herd immunity with its threshold.
- The chain of infection links the agent, its reservoir and source, a portal of exit, a mode of transmission, a portal of entry and a susceptible host; breaking any link stops spread.
- Carriers harbour the agent without symptoms and are often more dangerous than cases because they go unrecognised (typhoid, hepatitis B, diphtheria).
- Herd immunity protects the unimmunised when enough of the population is immune; the threshold is 1 − 1/R0 (very high, about 92–95%, for measles).
On this page
Overview
Infectious diseases spread through a series of linked steps: the chain of infection. Understanding each link shows where control measures can break the chain.
The chain of infection
- Infectious agent: bacteria, viruses, fungi, parasites, prions.
- Reservoir: where the agent normally lives and multiplies (humans, animals, soil).
- Portal of exit: respiratory tract, gut, urine, skin, blood, placenta.
- Mode of transmission: direct or indirect (below).
- Portal of entry: often the same route as the exit (respiratory, oral, skin, mucous membranes, injection).
- Susceptible host: someone without enough immunity.
Reservoir and source
- Reservoir: the natural home of the agent, where it survives and multiplies.
- Source: the person, animal or object from which the host actually acquires the infection.
Example: for hookworm, the reservoir is an infected human, but the source is soil contaminated with larvae.
Types of reservoir: human (cases and carriers), animal (diseases spread from animals to humans are zoonoses, for example rabies, brucellosis, anthrax) and non-living (soil: tetanus).
Carriers
A carrier is a person or animal that harbours the infectious agent without showing signs of disease and can spread it to others.
| Type | Meaning | Examples |
|---|---|---|
| Incubatory carrier | Spreads the infection during the incubation period, before symptoms | Measles, mumps, chickenpox, hepatitis B |
| Convalescent carrier | Continues to shed the agent during recovery | Typhoid, diphtheria, dysentery, cholera |
| Healthy (asymptomatic, contact) carrier | Has a subclinical infection and never gets ill | Polio, cholera, diphtheria, meningococcus |
| Temporary carrier | Sheds for a short time (under about 6 months) | |
| Chronic carrier | Sheds for a long time, sometimes for life | Typhoid (gallbladder), hepatitis B, malaria |
Carriers are often more dangerous than cases: they are not recognised, move about freely, and can spread infection for a long time. Typhoid Mary, a cook, is the classic example of a chronic typhoid carrier.
Modes of transmission
Direct transmission
- Direct contact: skin to skin, kissing, sexual contact (STIs, leprosy, scabies).
- Droplet infection: large droplets (over about 5 µm) from coughing, sneezing and talking; travel only about 1 metre (diphtheria, pertussis, meningococcus, influenza).
- Contact with soil (hookworm, tetanus).
- Inoculation into skin or mucosa (rabies by bite).
- Transplacental (vertical): from mother to fetus (rubella, syphilis, HIV, toxoplasmosis).
Indirect transmission
- Vehicle-borne: contaminated water, food, milk, blood, ice (cholera, typhoid, hepatitis A and E, polio).
- Vector-borne: insects or other animals.
- Mechanical: the agent is carried on the vector's body without multiplying (houseflies with diarrhoeal agents).
- Biological: the agent multiplies or develops inside the vector (malaria in Anopheles, dengue in Aedes). Biological transmission may be propagative (multiplication), cyclo-developmental (development only) or cyclo-propagative (both, as in malaria), and some agents pass to the vector's eggs (transovarian).
- Airborne:
- Droplet nuclei: tiny dried residues of droplets (under 5 µm) that stay in the air for long periods and travel far (tuberculosis, measles, chickenpox).
- Dust.
- Fomite-borne: contaminated objects such as towels, clothes, utensils, toys, door handles.
- Unclean hands and fingers.
Droplet nuclei (under 5 µm) cause airborne spread of TB, measles and chickenpox, needing airborne precautions and N95 masks.
Time periods
| Term | Meaning |
|---|---|
| Incubation period | From entry of the infection to the first sign or symptom |
| Latent period | From infection to the time the host becomes infectious (in non-infectious diseases, the time from exposure to diagnosis) |
| Communicable period | The time during which the infected person can transmit the infection |
| Generation time | From infection to the maximum infectivity of the host |
| Serial interval | The gap between the onset of symptoms in a primary case and in a secondary case |
Uses of the incubation period: tracing the source of an outbreak, deciding the length of quarantine and surveillance of contacts, and distinguishing point-source from propagated epidemics.
The incubation period ends with symptoms; the latent period (in infectious disease epidemiology) ends with infectiousness. A person can be infectious before symptoms appear (incubatory carrier).
Herd immunity
Herd immunity is the level of resistance of a community to an infection. When enough people are immune, the agent cannot find enough susceptible hosts, so transmission stops, and unimmunised people (such as young infants) are indirectly protected.
It depends on: the proportion immunised or naturally immune, the contagiousness of the disease, mixing in the population, and the presence of other hosts.
The basic reproduction number (R0) is the average number of secondary cases produced by one case in a fully susceptible population.
Herd immunity threshold = 1 − (1 ÷ R0)
Measles: R0 about 15 → threshold = 1 − 1/15 = 1 − 0.067 = 0.93 (93%)
A disease with R0 = 4 → threshold = 1 − 1/4 = 0.75 (75%)
The more contagious the disease, the higher the coverage needed
Herd immunity does not work for tetanus, because tetanus does not spread from person to person.
Breaking the chain
- Agent and reservoir: treat cases and carriers, control animal reservoirs.
- Transmission: clean water, sanitation, food safety, hand hygiene, vector control, masks, isolation.
- Susceptible host: immunisation, chemoprophylaxis, nutrition.
Expect: the six links of the chain of infection, reservoir vs source, carrier types with examples, why carriers are dangerous, droplet vs droplet nuclei, mechanical vs biological vectors, fomites, incubation vs latent vs communicable periods, and herd immunity with its threshold and exceptions.
Quick revision
- Chain: agent → reservoir → portal of exit → transmission → portal of entry → susceptible host.
- Reservoir: where it lives; source: where the host gets it.
- Carriers: incubatory, convalescent, healthy; chronic typhoid and hepatitis B.
- Droplets travel about 1 m; droplet nuclei (under 5 µm) are airborne (TB, measles).
- Incubation: infection to symptoms; communicable period: when it spreads.
- Herd immunity threshold = 1 − 1/R0; not for tetanus.
Written by The House Job from Excel Community Medicine and Park's Textbook of Preventive and Social Medicine (named in the UHS PGET syllabus).
FAQ
What is a carrier?
A person or animal that harbours an infectious agent without symptoms and can transmit it to others.
What is the difference between a reservoir and a source of infection?
The reservoir is where the agent normally lives and multiplies; the source is where the host actually acquires it.
What is the incubation period?
The time from entry of the infection to the appearance of the first sign or symptom.
What is herd immunity?
The resistance of a community to an infection when enough people are immune that transmission stops, protecting those who are not immune.