Disease transmission is the process by which an infectious agent, or pathogen, leaves its reservoir or host and is conveyed to a new, susceptible host. Pathogens, which include viruses, bacteria, fungi, and parasites, require a pathway to bridge the gap between an infected source and a susceptible person. Understanding these pathways is fundamental to public health, as interrupting the specific mode of transmission is the most effective way to prevent the spread of illness. Transmission methods are generally categorized by the nature of the physical link involved, ranging from immediate physical contact to travel via air or an intermediary organism.
Direct Contact Transmission
Direct contact transmission involves the immediate physical transfer of a pathogen from an infected person or animal to a susceptible individual. This method requires close proximity, often involving touching, kissing, or sexual contact, allowing microorganisms to pass directly between hosts. Person-to-person contact is a common route for illnesses such as the common cold, mononucleosis, and sexually transmitted infections (STIs) like HIV and Hepatitis B, which are passed through bodily fluids.
Another form of direct transmission is the short-range droplet spray produced when an infected person coughs, sneezes, or talks loudly. These relatively large droplets travel only a very short distance, typically less than three to six feet, before falling quickly to the ground. Infection occurs when these droplets land directly onto the mucous membranes of another person’s eyes, nose, or mouth. Diseases like pertussis and meningococcal infection are often spread through this immediate, face-to-face droplet exposure.
Indirect Contact Transmission
Indirect contact transmission occurs when a pathogen is transferred to a susceptible person via an inanimate object or surface. These contaminated objects, known as fomites, serve as temporary carriers for the infectious agent. Common examples of fomites include doorknobs, light switches, computer keyboards, and shared medical instruments.
The process begins when an infected person deposits the pathogen onto a surface, where it survives for a period of time. A new host becomes infected by touching the contaminated fomite and subsequently touching a portal of entry on their own body, such as their mouth, nose, or eyes. Certain types of influenza viruses and gastrointestinal illnesses like Norovirus can spread this way, persisting on surfaces until picked up by a new host. In healthcare settings, inadequate sterilization of equipment can also lead to indirect transmission.
Respiratory Spread via Droplets and Aerosols
Respiratory spread involves the expulsion of infectious particles from the respiratory tract of an infected person into the air. The distinction in this category lies in the size and behavior of the expelled particles, which are categorized as droplets or aerosols. Droplets are the larger respiratory particles, measuring greater than 5 to 10 micrometers in diameter, and they fall rapidly from the air, usually within the first few feet of travel.
Aerosols, or droplet nuclei, are much smaller particles, typically less than 5 micrometers, that form when the water content of a larger droplet evaporates rapidly. Due to their small size and light weight, aerosols can remain suspended in the air for extended periods and are capable of traveling long distances on air currents. This mechanism, often referred to as airborne transmission, allows them to infect people who are not in close proximity to the original source. Diseases like the seasonal flu spread via larger droplets during close contact, while illnesses such as measles and pulmonary tuberculosis spread effectively through the long-range airborne route via aerosols.
Vehicle and Vector Transmission
Transmission that occurs through non-human, non-respiratory intermediaries is divided into vehicle and vector transmission. Vehicle transmission involves a non-living substance or object that serves as a common medium for infection for multiple people. The most common vehicles are contaminated water, food, and biological products like blood.
Contaminated water can spread waterborne diseases like cholera, while improperly handled food can cause outbreaks of foodborne illnesses, such as Salmonella. Vehicle transmission also occurs through contaminated injectable substances, such as shared needles, which transmit bloodborne pathogens like Hepatitis C. In contrast, vector transmission involves a living organism, typically an arthropod such as a mosquito, tick, or flea, that carries the pathogen between hosts.
Mechanical and Biological Vectors
The vector’s role further distinguishes this living transfer in the pathogen’s life cycle. Mechanical vector transmission occurs when the arthropod passively carries the infectious agent on its body parts, such as a housefly carrying bacteria from waste to food. Biological vector transmission is more complex, as the pathogen must multiply or undergo developmental changes inside the vector before transmission. Mosquitoes transmitting malaria or ticks transmitting the bacteria that causes Lyme disease are prime examples of this biological process.