When a pathogen spreads from an infected source to a new, susceptible host, this process is known as disease transmission. Understanding how these infectious agents move is fundamental to public health and epidemiology, as interrupting transmission is the most effective way to prevent outbreaks. Pathogens must travel from their reservoir to a host through a specific route, often described as a chain of infection. Scientists have grouped these primary routes into five distinct modes of transmission.
Contact Transmission
Contact transmission involves the physical transfer of an infectious agent and is divided into two forms: direct and indirect. Direct contact occurs when there is an immediate physical connection between the infected and susceptible host. Examples include skin-to-skin touching, kissing, sexual intercourse, or contact with soil harboring infectious organisms.
Direct contact also includes the spread of pathogens through blood or other bodily fluids. Indirect contact involves a contaminated intermediate object or substance. An inanimate object that passively carries a pathogen is called a fomite, such as a doorknob or shared medical equipment.
Infection occurs when a person touches the fomite and then transfers the pathogen to a mucous membrane, like the eye, nose, or mouth. Indirect contact via fomites is a common means of spread for many respiratory and gastrointestinal illnesses.
Droplet Transmission
Droplet transmission is a form of respiratory spread involving the expulsion of relatively large, moist particles from the respiratory tract of an infected person. These particles are produced when someone coughs, sneezes, or talks, and are typically greater than 5 micrometers (µm) in diameter.
Due to their mass, these droplets cannot remain suspended in the air for long and travel only short distances before falling to the ground. Transmission generally requires close proximity, usually within 3 to 6 feet (1 to 2 meters), for exposure. Infection occurs when these droplets land directly on the susceptible host’s mucous membranes.
Airborne Transmission
Airborne transmission involves the spread of pathogens through much smaller particles known as aerosolized droplet nuclei. These particles are residues from evaporated droplets and are 5 micrometers (µm) or less in diameter. Their minute size allows them to remain suspended in the air for extended periods and travel long distances on air currents.
Unlike large droplets that fall quickly, these aerosolized particles can circulate throughout a room or building, leading to infection even after the source has left. Pathogens transmitted this way, such as the measles virus, can be inhaled deep into the lower respiratory tract. This mechanism often necessitates specialized ventilation controls to limit the circulation of infectious aerosols.
Vehicle Transmission
Vehicle transmission occurs when a single, non-living source, or vehicle, becomes contaminated and transmits the infectious agent to multiple hosts. This mode is often responsible for large-scale, common-source outbreaks because the vehicle passively transports the pathogen from its reservoir to the susceptible population.
Contaminated food is a frequent vehicle, leading to foodborne illness when pathogens like E. coli or Salmonella are ingested. Contaminated water is another widespread vehicle for waterborne diseases, such as cholera, especially in areas with poor sanitation. Shared medical supplies, intravenous fluids, blood products, or improperly sterilized equipment can also serve as vehicles for transmission in healthcare settings.
Vector Transmission
Vector transmission involves the spread of an infectious agent through a living organism, typically an arthropod like a mosquito, tick, or fly, which is known as a vector. This mode is categorized based on the vector’s role in the pathogen’s life cycle.
Mechanical Vectors
A mechanical vector simply carries the pathogen on the outside of its body, such as on its feet, without the agent reproducing inside it. A housefly landing on contaminated feces and then on human food is a classic example.
Biological Vectors
A biological vector plays an active role in the pathogen’s life cycle. The infectious agent reproduces or undergoes a developmental change inside the vector before being transmitted to the host, often through a bite. Mosquitoes transmitting the parasite that causes malaria and ticks spreading the bacteria responsible for Lyme disease are prime examples. Controlling these populations is a major public health strategy.