The term “transmissible” describes the ability of a disease, condition, or agent to be passed from one living organism to another. The focus is purely on the capacity for transfer, without specifying the method or ease of such a process.
How Transmission Occurs
The spread of transmissible agents can happen through several distinct pathways. One common method is direct contact, involving physical interaction between an infected individual and a susceptible person. This can include touching, kissing, or skin-to-skin contact, where microorganisms like those causing impetigo can transfer directly.
Another significant pathway involves respiratory droplets or airborne particles. When an infected person coughs, sneezes, or talks, tiny droplets containing pathogens can be expelled into the air. These droplets can then be inhaled by others nearby, a common way illnesses like influenza spread. Smaller, aerosolized particles can remain suspended in the air for longer periods and travel greater distances.
Indirect contact transmission occurs when an agent is transferred via an inanimate object, often referred to as a fomite. This happens when an infected person touches a surface, leaving behind pathogens that another person then picks up by touching the same surface and subsequently touching their eyes, nose, or mouth. Contaminated doorknobs or shared computer keyboards are examples of such surfaces.
Vector-borne transmission involves an intermediate organism, typically an arthropod like a mosquito or tick, carrying the pathogen from one host to another. For instance, mosquitoes can transmit the West Nile virus by biting an infected animal and then biting a human, thereby transferring the virus. This mechanism bypasses direct person-to-person spread.
Consumption of contaminated food or water represents another pathway for transmission. Pathogens like Salmonella or E. coli can enter the body when individuals ingest food or water that has been improperly handled or treated.
What Can Be Transmitted
A variety of microscopic agents possess the ability to be transmitted between organisms. Viruses are among these, consisting of genetic material enclosed within a protein shell, which replicate by invading living cells. The influenza virus, for example, spreads by hijacking human cells to produce more viral particles.
Bacteria are single-celled organisms that can cause illness through various mechanisms, including producing toxins or directly damaging tissues. Streptococcus pyogenes, a common bacterium, is responsible for conditions like strep throat and can be transmitted through respiratory droplets.
Fungi, which include yeasts, molds, and mushrooms, can also be transmitted and cause infections. Organisms like Trichophyton rubrum are responsible for common skin conditions such as ringworm and athlete’s foot. These fungal spores can spread through direct contact or by touching contaminated surfaces.
Parasites are organisms that live on or in a host, obtaining nutrients at the host’s expense. Giardia lamblia, a microscopic parasite, can cause diarrheal disease and is often transmitted through contaminated water or food.
Transmissible vs. Infectious vs. Contagious
“Transmissible” is the broadest of these terms, simply indicating that something has the capacity to be passed from one entity to another. This term applies to any agent or condition that can move between hosts, regardless of the mechanism or ease of transfer.
“Infectious” refers specifically to diseases caused by pathogenic microorganisms that invade the body and multiply. While most infectious diseases are transmissible, not all infectious diseases are transmissible between individuals. For example, tetanus is an infectious disease caused by Clostridium tetani bacteria entering a wound, but it does not spread from person to person.
“Contagious” describes a subset of transmissible diseases that spread very easily from one person to another. This often occurs through direct contact, respiratory droplets, or airborne transmission. Measles, for instance, is highly contagious because its virus can remain airborne for extended periods and infect susceptible individuals who enter the same space. All contagious diseases are transmissible, but not all transmissible diseases exhibit the high degree of easy person-to-person spread that defines contagiousness.