Pathogen transmission

In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previously infected.^[1] The term strictly refers to the transmission of microorganisms directly from one individual to another by one or more of the following means:

airborne transmission – very small dry and wet particles that stay in the air for long periods of time allowing airborne contamination even after the departure of the host. Particle size < 5 μm.
droplet transmission – small and usually wet particles that stay in the air for a short period of time. Contamination usually occurs in the presence of the host. Particle size > 5 μm.
direct physical contact – touching an infected individual, including sexual contact
indirect physical contact – usually by touching a contaminated surface, including soil (fomite)
fecal–oral transmission – usually from unwashed hands, contaminated food or water sources due to lack of sanitation and hygiene, an important transmission route in pediatrics, veterinary medicine and developing countries.
via contaminated hypodermic needles or blood products

Transmission can also be indirect, via another organism, either a vector (e.g. a mosquito or fly) or an intermediate host (e.g. tapeworm in pigs can be transmitted to humans who ingest improperly cooked pork). Indirect transmission could involve zoonoses or, more typically, larger pathogens like macroparasites with more complex life cycles. Transmissions can be autochthonous (i.e. between two individuals in the same place) or may involve travel of the microorganism or the affected hosts.

A 2024 World Health Organization report standardized the terminology for the transmission modes of all respiratory pathogens in alignment with particle physics: airborne transmission; inhalation; direct deposition; and contact.^[2] But these newly standardized terms have yet to be translated to policy, including infection control policy^[2] or the pandemic accords or updated International Health Regulations.

^ Bush AO, Fernández JC, Esch GW, Seed JR (2001). Parasitism: The Diversity and Ecology of Animal Parasites. Cambridge, UK: Cambridge University Press. pp. 391–399. ISBN 9780521664479.
^ ^a ^b Leung NH, Milton DK (3 July 2024). "New WHO proposed terminology for respiratory pathogen transmission". Nature Reviews Microbiology: 1–2. doi:10.1038/s41579-024-01067-5. ISSN 1740-1534.

[1] Bush AO, Fernández JC, Esch GW, Seed JR (2001). Parasitism: The Diversity and Ecology of Animal Parasites. Cambridge, UK: Cambridge University Press. pp. 391–399. ISBN 9780521664479.

[:4-2] Leung NH, Milton DK (3 July 2024). "New WHO proposed terminology for respiratory pathogen transmission". Nature Reviews Microbiology: 1–2. doi:10.1038/s41579-024-01067-5. ISSN 1740-1534.

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