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. 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
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 or an intermediate host. Indirect transmission could involve zoonoses or, more typically, larger pathogens like macroparasites with more complex life cycles. Transmissions can be autochthonous 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. But these newly standardized terms have yet to be translated to policy, including infection control policy or the pandemic accords or updated International Health Regulations.

Definition and related terms

An infectious disease agent can be transmitted in two ways: as disease agent transmission from one individual to another in the same generation by either direct contact, or indirect contact through air – cough or sneeze
or by vertical disease transmission, passing the agent causing the disease from parent to offspring, such as in prenatal or perinatal transmission.
The term infectivity describes the ability of an organism to enter, survive and multiply in the host, while the infectiousness of a disease agent indicates the comparative ease with which the disease agent is transmitted to other hosts. Transmission of pathogens can occur by direct contact, through contaminated food, body fluids or objects, by airborne inhalation or through vector organisms.
Transmissibility is the probability of an infection, given a contact between an infected host and a noninfected host.
Community transmission means that the source of infection for the spread of an illness is unknown or a link in terms of contacts between patients and other people is missing. It refers to the difficulty in grasping the epidemiological link in the community beyond confirmed cases.
Local transmission means that the source of the infection has been identified within the reporting location.

Routes of transmission

The route of transmission is important to epidemiologists because patterns of contact vary between different populations and different groups of populations depending on socio-economic, cultural and other features. For example, low personal and food hygiene due to the lack of a clean water supply may result in increased transmission of diseases by the fecal-oral route, such as cholera. Differences in incidence of such diseases between different groups can also throw light on the routes of transmission of the disease. For example, if it is noted that polio is more common in cities in underdeveloped countries, without a clean water supply, than in cities with a good plumbing system, we might advance the theory that polio is spread by the fecal-oral route. Two routes are considered to be airborne: Airborne infections and droplet infections.

Airborne infection

"Airborne transmission refers to infectious agents that are spread via droplet nuclei containing infective microorganisms. These organisms can survive outside the body and remain suspended in the air for long periods of time. They infect others via the upper and lower respiratory tracts." The size of the particles for airborne infections need to be < 5 μm. It includes both dry and wet aerosols and thus requires usually higher levels of isolation since it can stay suspended in the air for longer periods of time. i.e., separate ventilation systems or negative pressure environments are needed to avoid general contamination. e.g., tuberculosis, chickenpox, measles.

infection

A common form of transmission is by way of respiratory droplets, generated by coughing, sneezing, or talking. Respiratory droplet transmission is the usual route for respiratory infections. Transmission can occur when respiratory droplets reach susceptible mucosal surfaces, such as in the eyes, nose or mouth. This can also happen indirectly via contact with contaminated surfaces when hands then touch the face. Before drying, respiratory droplets are large and cannot remain suspended in the air for long, and are usually dispersed over short distances. The size of the particles for droplet infections are > 5 μm.
Organisms spread by droplet transmission include respiratory viruses such as influenza virus, parainfluenza virus, adenoviruses, rhinovirus, respiratory syncytial virus, human metapneumovirus, Bordetella pertussis, pneumococci, streptococcus pyogenes, diphtheria, rubella, and coronaviruses. Spread of respiratory droplets from the wearer can be reduced through wearing of a surgical mask.

Direct contact

Direct contact occurs through skin-to-skin contact, kissing, and sexual intercourse. Direct contact also refers to contact with soil or vegetation harboring infectious organisms. Additionally, while fecal–oral transmission is primarily considered an indirect contact route, direct contact can also result in transmission through feces.
Diseases that can be transmitted by direct contact are called contagious. These diseases can also be transmitted by sharing a towel or items of clothing in close contact with the body if they are not washed thoroughly between uses. For this reason, contagious diseases often break out in schools, where towels are shared and personal items of clothing accidentally swapped in the changing rooms.
Some diseases that are transmissible by direct contact include athlete's foot, impetigo, syphilis, warts, and conjunctivitis.

Sexual

This refers to any infection that can be caught during sexual activity with another person, including vaginal or anal sex, less commonly through oral sex and rarely through manual sex. Transmission is either directly between surfaces in contact during intercourse or from secretions which carry infectious agents that get into the partner's blood stream through tiny tears in the penis, vagina or rectum. In this second case, anal sex is considerably more hazardous since the penis opens more tears in the rectum than the vagina, as the vagina is more elastic and more accommodating.
Some infections transmissible by the sexual route include HIV/AIDS, chlamydia, genital warts, gonorrhea, hepatitis B, syphilis, herpes, and trichomoniasis.

Oral sex

s such as HIV and hepatitis B are thought to not normally be transmitted through mouth-to-mouth contact, although it is possible to transmit some STIs between the genitals and the mouth, during oral sex. In the case of HIV, this possibility has been established. It is also responsible for the increased incidence of herpes simplex virus 1 in genital infections and the increased incidence of the type 2 virus in oral infections.

Manual sex

While rare in regards to this sexual practice, some infections that can spread via manual sex include HPV, chlamydia, and syphilis.

Oral

Infections that are transmitted primarily by oral means may be caught through direct oral contact such as kissing, or by indirect contact such as by sharing a drinking glass or a cigarette. Infections that are known to be transmissible by kissing or by other direct or indirect oral contact include all of the infections transmissible by droplet contact and all forms of herpes viruses, namely Cytomegalovirus infections herpes simplex virus and infectious mononucleosis.

Mother-to-child transmission

This is from mother to child, often in utero, during childbirth or during postnatal physical contact between parents and offspring. In mammals, including humans, it occurs also via breast milk. Infectious diseases that can be transmitted in this way include: HIV, hepatitis B and syphilis. Many mutualistic organisms are transmitted vertically.

Iatrogenic

Transmission due to medical procedures, such as touching a wound, the use of contaminated medical equipment, or an injection or transplantation of infected material. Some diseases that can be transmitted iatrogenically include Creutzfeldt–Jakob disease, HIV, and many more.

Needle sharing

This is the practice of intravenous drug-users by which a needle or syringe is shared by multiple individuals to administer intravenous drugs such as heroin, steroids, and hormones. This can act as a vector for blood-borne diseases, such as Hepatitis C and HIV.

Indirect contact

Indirect contact transmission, also known as vehicle-borne transmission, involves transmission through contamination of inanimate objects. Vehicles that may indirectly transmit an infectious agent include food, water, biologic products such as blood, and fomites such as handkerchiefs, bedding, or surgical scalpels. A vehicle may passively carry a pathogen, as in the case of food or water may carrying hepatitis A virus. Alternatively, the vehicle may provide an environment in which the agent grows, multiplies, or produces toxin, such as improperly canned foods provide an environment that supports production of botulinum toxin by Clostridium botulinum.