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What are the five ways infectious diseases can be spread?
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What are the five ways infectious diseases can be spread?

Written by
Kara Rogers
Kara Rogers is the senior editor of biomedical sciences at Encyclopædia Britannica, where she oversees a range of content from medicine and genetics to microorganisms. She joined Britannica in 2006 and...
Kara Rogers
Fact-checked by
Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
Britannica Editors
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•History

There are five primary ways in which infectious diseases can be spread: direct contact, contact with fomites (inanimate objects and surfaces), airborne transmission, droplet transmission, and vector-borne transmission. Less common modes include vertical transmission (from mother to child) and transfusion transmission, when contaminated donor blood gives rise to an infection in the transfusion recipient.

Primary modes of transmission

  • Direct contact: an infectious agent is passed from one individual to another via touching, kissing, sexual contact, or contact with infected body fluids.
  • Fomite contact: an infectious agent moves from an infected individual to another through contact with a contaminated surface or inanimate object, such as a doorknob or keyboard.
  • Airborne transmission: small, aerosolized particles of an infectious agent released through breathing, coughing, sneezing, or speaking are inhaled by another individual.
  • Droplet transmission: large, heavy droplets containing an infectious agent, expelled primarily through coughing or sneezing, land on the eyes, nose, or mouth of another individual.
  • Vector-borne transmission: an infectious agent passes from one individual to another through a vector, such as a mosquito or tick. Vectors transmit pathogens between humans or between animals and humans.

Pathogens and mechanisms of infection

Different pathogens rely on different forms of transmission. Herpesviruses, for example, move to uninfected individuals via direct contact with bodily fluids (particularly saliva) or with the skin or mucous membranes of infected persons, which can include asymptomatic carriers. Human immunodeficiency virus (HIV), the cause of AIDS, is transmitted exclusively through direct contact with specific body fluids, namely blood, breast milk, rectal fluids, semen, and vaginal fluids. Many kinds of infectious agents are spread through contact with fomites, including influenza viruses, norovirus, and rotavirus. Fomites also are a common route of fecal-oral transmission, in which infectious fecal matter from contaminated objects or surfaces enters the body through the oral cavity.

Many agents that cause respiratory infections, such as influenza, measles, SARS-CoV-2 (the cause of COVID-19), and tuberculosis, are transmitted as aerosolized particles through the air. Likewise, the agents that cause illnesses such as COVID-19, influenza, mumps, pertussis, and strep throat can be transmitted as heavy droplets that land on mucous membranes.

In vector-borne transmission, infection may be biological or mechanical in nature. In biological vectors, the infectious agent passes through a phase of change prior to its transmission to a new host. An example is Lyme disease, in which the causative bacterium undergoes changes in the expression of an antigen known as outer surface protein A (OspA) when the vector (a deer tick) feeds on host blood. Mechanical vectors passively pass infectious agents from one host or surface to another; the pathogen does not develop or replicate inside the vector. An example of a mechanical vector is the housefly (Musca domestica), which can pick up bacteria such as E. coli or Salmonella from waste; when the fly lands on food, it deposits some of the bacteria, leading to infection and illness when the food is consumed.

Kara Rogers