Transmission of Infection

Transfer of the organism from an infected host to a susceptible host without an intermediate carrier or vehicle.

• Contact: direct physical touch between an infected and susceptible individual (skin-to-skin, e.g. impetigo, cutaneous herpes simplex).
• Sexual transmission: direct contact during sexual activity (Neisseria gonorrhoeae, Chlamydia trachomatis, Treponema pallidum, HIV, herpes simplex virus).
• Droplet spread: expulsion of respiratory secretions (coughing, sneezing, talking) as large droplets that travel a limited distance (typically under 1-2 metres) before settling, landing directly on mucosal surfaces of a nearby susceptible host (influenza, Bordetella pertussis, Neisseria meningitidis). Distinct from true airborne transmission (below) by droplet size and travel distance.
• Vertical transmission: transfer from mother to offspring, covered in its own dedicated section below given its clinical importance.

Transfer via an intermediate vehicle or vector rather than direct host-to-host contact.

• Fomites: inanimate objects (doorknobs, medical equipment, shared utensils) contaminated with an organism, transmitting infection when touched and then transferred to a mucosal surface.
• Food: ingestion of contaminated food (Salmonella, Listeria monocytogenes, hepatitis A virus, various helminths via undercooked meat or fish).
• Water: ingestion of or contact with contaminated water (Vibrio cholerae, Cryptosporidium, Giardia lamblia, Schistosoma via skin penetration during water contact).
• Airborne spread: transmission via small droplet nuclei (under 5 micrometres) that remain suspended in air for extended periods and travel substantial distances beyond the immediate vicinity of the source, distinct from droplet spread. Classic examples: Mycobacterium tuberculosis, measles virus, varicella-zoster virus. This distinction underlies the difference between droplet precautions and airborne precautions in infection control (full detail in Sterilisation, Disinfection and Infection Prevention).
• Vectors: a living organism, almost always an arthropod, that actively transmits the pathogen between hosts (mosquitoes for malaria and arboviruses, ticks for Lyme disease and rickettsial infections, tsetse flies for African trypanosomiasis).

Transfer from mother to offspring, via three possible routes:

• Transplacental: organism crosses the placenta directly into the fetal circulation during pregnancy (TORCH organisms: Toxoplasma gondii, rubella virus, cytomegalovirus, herpes simplex virus, and others including Treponema pallidum, parvovirus B19, Zika virus, HIV, varicella-zoster virus).
• During delivery: exposure to maternal genital tract secretions or blood during passage through the birth canal (Neisseria gonorrhoeae and Chlamydia trachomatis, causing neonatal conjunctivitis; group B Streptococcus, causing neonatal sepsis; herpes simplex virus, if active maternal genital lesions are present at delivery; hepatitis B virus).
• Breastfeeding: transmission via breast milk (HIV, human T-lymphotropic virus type 1, cytomegalovirus in preterm infants).

• Animal reservoirs: many pathogens are maintained primarily in an animal population, with humans as an incidental or secondary host (Salmonella from poultry and reptiles, Brucella from livestock, rabies virus from multiple mammalian reservoirs, hantavirus from rodents).
• Vector-borne zoonoses: infections maintained in an animal reservoir and transmitted to humans via an arthropod vector rather than direct contact (Lyme disease, maintained in a rodent/deer reservoir and transmitted via Ixodes ticks; plague, maintained in rodent populations and transmitted via fleas).
• Full organism-specific zoonotic detail is covered in Public Health and Epidemiological Microbiology.

The six linked elements that must all be present, in sequence, for transmission to occur. Interrupting any single link prevents transmission, the conceptual basis of most infection control interventions.

• Infection control measures each target a specific link: hand hygiene interrupts the mode of transmission; vaccination reduces the pool of susceptible hosts; isolation precautions block the portal of exit or the mode of transmission; source control (covering a cough, safe sharps disposal) blocks the portal of exit directly.