An epidemic is the outbreak of an infectious disease in many people in a region, more than the “normal” disease occurrence. This happens in short period of time (approximately 2 weeks or less). People who don’t have a developed or inherent immunity is affected.

Many factors commonly trigger infectious disease epidemics, including:

  • A host population’s change in biology (increased strain or population of a disease carrier).
  • A pathogen reservoir* genetic change.
  • Introduction of mutating* pathogens to a host population – caused by the movement of the host population or pathogen.

For human diseases, the body is the host of infection. Ebola, Typhoid Fever, Smallpox, and Malaria are some epidemics that South Africa encountered. Organisms can spread in many ways in an epidemic. The reservoir may be human, animal or inanimate objects.

Animal reservoirs spread disease to humans: Yellow fever, spread by the Aedes mosquito; Lyme disease, spread by ticks; rabies, spread by bats, cats, and dogs.

Inanimate reservoirs– drinking water contaminated by human/animal feces are an environment for organisms causing gastrointestinal diseases.

Although signs and symptoms of a disease are obvious, some carry and spread disease without any signs or symptoms during the symptomless incubation period, or convalescent period (recovering time). This fuels the epidemic, since there isn’t a reason for prevention precautions.

Infected people become reservoirs that spread the disease by Contact Transmission in 2 ways:

  • Direct Contact Transmission– when infected hosts interact with susceptible people; by touching, kissing, and sexual intercourse- transmitting influenza, smallpox, STDs, AIDS etc.
  • Indirect Contact Transmission– the disease-causing organismis spread from the reservoir to a vulnerable host by fomite, a non-living carrier like towels, needles or eating utensils.


Epidemics can be transmitted through food, water, air, and blood:

Droplet Transmission– mucous spread microorganisms that travel less than 0.9m from the mouth/nose during coughing, sneezing or talking; spreading influenza and pneumonia.

Waterborne Transmission– contaminated water causes epidemic (like cholera) transmission.

Foodborne Poisoning– occurs when food is improperly cooked, unrefrigerated, or prepared by infected people.

Airborne Transmission– viral diseases like measles and tuberculosis occur when infectious organisms travel from human to host by spray from the mouth/nose. It spreads fungal infections because spores travel on dust particles.


Vectors, usually insects, carry pathogens from host to host, which spread an epidemic by mechanical or biological transmission:

Mechanical Transmission– flies transfer organisms causing typhoid fever from the infected feces to food, the disease is spread.

Biological Transmission– arthropods bite a host, ingesting infected blood, disease-causing organisms reproduce in the gut, increasing parasite transmittance to the next host. In a bitten host parasites are passed into a wound when the vector passes feces or vomits; like malaria spread by the Anopheles mosquito vector.


After epidemic introduction, infection spreads quickly in vulnerable hosts, increasing occurrence over time, until a maximum is reached. The epidemic then subsides due to the lack of vulnerable individuals- most have already gained immunity called herd immunity, which tends to disappear over time due to:

(1) deterioration of individual immunity.

(2) death of immune people.

(3) arrival of vulnerable people by birth or emigration epidemic areas.







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