Infectious diseases spread when a pathogen moves from an infected or colonised source to a susceptible host through a route that permits successful infection. Transmission can occur through direct contact, indirect contact, respiratory particles or aerosols, contaminated food or water, blood and other body fluids, sexual contact, vectors such as mosquitoes and ticks, or vertical transmission from parent to child. The exact route depends on the pathogen's biology and where it is present in the body.
Transmission is influenced by infectiousness, the amount of pathogen shed, the duration of infectiousness, environmental survival, host susceptibility, behaviour, population density and opportunities for contact. Not every person exposed to an infectious agent becomes infected, and not every infected person develops symptoms. Spread is therefore a probabilistic process governed by interactions among pathogen, host and environment.
Direct contact transmission occurs when an infectious agent passes through physical contact between an infected person and another person. Examples include transmission through touching infected skin lesions or contact with secretions. Sexual transmission is a specialised form of direct or body-fluid exposure and can involve pathogens such as HIV, herpes simplex viruses, gonorrhoea-causing bacteria and HPV. Indirect contact transmission occurs when pathogens are transferred through contaminated objects or surfaces, sometimes called fomites.
The importance of surface transmission varies considerably between pathogens because organisms differ in their ability to survive outside the body and reach susceptible tissues. Hand hygiene can reduce transmission of many pathogens because hands frequently connect contaminated surfaces, body sites and the mouth, nose or eyes.
Respiratory transmission occurs when infectious material from the respiratory tract reaches another person's respiratory system. Larger respiratory droplets and smaller airborne particles can behave differently depending on their size, environmental conditions and the circumstances of release. Speaking, coughing, sneezing, singing and breathing can generate particles, although the quantity and composition vary. Some pathogens can remain suspended in indoor air sufficiently long to permit airborne transmission, while others rely more heavily on close-range droplets or direct contact.
Ventilation can reduce concentrations of infectious particles indoors by replacing contaminated air with cleaner air. Masking, staying away from others while infectious and improving ventilation can therefore reduce transmission for pathogens that spread through respiratory routes, although their effectiveness depends on correct use and the specific pathogen.
Foodborne and waterborne transmission occurs when pathogens are ingested through contaminated food or water. Contamination can occur during production, processing, preparation, storage or handling, and inadequate sanitation can allow faecal organisms to enter water supplies or food chains. Pathogens such as norovirus, Salmonella, Campylobacter, pathogenic Escherichia coli and Vibrio species can spread through food or water under appropriate circumstances. Some organisms produce toxins in food, while others infect the gastrointestinal tract after ingestion.
Proper cooking, refrigeration, hand hygiene, safe water treatment, separation of raw and cooked foods and effective sanitation can interrupt these transmission pathways. The infectious dose varies between pathogens, meaning that some require relatively large exposures while others can cause infection after a much smaller inoculum.
Bloodborne transmission occurs when pathogens are transferred through infected blood or certain body fluids, particularly when these fluids enter another person's bloodstream or susceptible tissues. Shared injecting equipment can transmit bloodborne viruses, while healthcare-associated transmission can occur through unsafe equipment or inadequate infection-control practices. Sexual exposure can also transmit pathogens through mucosal surfaces and body fluids. Vector-borne transmission involves an organism such as a mosquito, tick or flea acquiring a pathogen from one host and transmitting it to another. Malaria is transmitted by Anopheles mosquitoes, while Lyme disease is transmitted by infected ticks. Vector control, environmental management, insecticide-treated materials and other measures can reduce transmission depending on the disease and vector.
Vertical transmission occurs when an infectious agent passes from a pregnant person to a fetus or from a parent to an infant around birth or through breastfeeding, depending on the pathogen. Transmission can occur across the placenta, during labour and delivery, or after birth. Examples include congenital infections caused by pathogens such as rubella virus, cytomegalovirus, syphilis and certain other organisms. Some infections can also be transmitted through breast milk. Prevention strategies vary and can include vaccination before pregnancy, antenatal screening, maternal treatment, preventive medicines, delivery interventions or specialised newborn care. The route and timing of transmission matter because the same pathogen can produce very different outcomes depending on when infection occurs during development.
Transmission is influenced by the concept of the chain of infection, which includes the infectious agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host. Breaking any link can reduce spread. Treating an infected person can reduce pathogen load and infectiousness; covering respiratory secretions can reduce release; hand hygiene can interrupt contact transmission; sanitation can interrupt faecal-oral transmission; vaccination reduces the pool of susceptible hosts; and vector control can interrupt transmission by arthropods.
Isolation separates people who are known or suspected to be infected from others, while quarantine separates people who have been exposed and may subsequently become infectious. These terms are therefore related but have different meanings. Effective infection control usually combines several interventions rather than relying on one measure.
A person can sometimes transmit an infection before symptoms develop or while having no symptoms, depending on the pathogen. The infectious period is the interval during which an infected person can transmit the pathogen, and it may not coincide with the period of symptoms. Asymptomatic or presymptomatic transmission can make control more difficult because people may not realise that they are infectious.
Superspreading events can occur when particular individuals, settings or circumstances generate disproportionately many transmission events, often because of high pathogen shedding combined with numerous close contacts or environmental conditions. Transmission also varies according to season, population immunity, behaviour and geography. Understanding these dynamics is central to epidemiology because interventions must target the actual route and timing of transmission rather than assuming that all infectious diseases spread in the same way.
