A virus is a microscopic infectious agent that carries its own genetic material. Because viruses lack the internal machinery to reproduce independently, they must invade a living host cell, hijacking its components to manufacture thousands of new copies. For a virus to survive and persist, it must be effectively transferred from an infected individual to a new, susceptible host, completing the transmission cycle. The specific physical pathway a virus takes to move between people determines its success, and these distinct routes are organized into several major categories.
Direct Contact Transmission
Direct contact transmission involves the immediate physical transfer of the virus between an infected and a non-infected person, requiring close proximity. This occurs through interactions such as touching, kissing, or sexual intercourse, allowing the virus to be shed from an infected area and directly enter the new host’s body. Viruses that infect the skin, like papillomavirus, are spread through skin-to-skin contact, while others are transmitted through saliva during kissing. Sexual transmission involves the exchange of bodily fluids like blood, semen, or vaginal secretions, which is the primary route for viruses such as Human Immunodeficiency Virus (HIV). Another element is vertical transmission, where the virus passes from a mother to her child during pregnancy, the birthing process, or shortly after birth through breastfeeding.
Transmission Via Respiratory Particles
Many viruses that cause respiratory illnesses are transmitted when an infected person expels respiratory particles through coughing, sneezing, talking, or breathing. The distinction between these particles is based primarily on size, which dictates how far they travel and how long they remain suspended in the air. Droplet transmission involves larger particles that are too heavy to remain airborne for long, typically falling onto nearby surfaces within a short distance. Infection occurs when these droplets land directly on the mucosal surfaces—the eyes, nose, or mouth—of a nearby individual. Airborne transmission involves much smaller particles, often called droplet nuclei, which can remain suspended in the air for extended periods and travel over longer distances on air currents, infecting a susceptible person even after the infected individual has left the area.
Transmission Through Contaminated Surfaces (Fomites)
Indirect transmission occurs when a virus is transferred via an inanimate object that serves as an intermediate carrier, known as a fomite. Fomites are common objects like doorknobs, shared equipment, or tabletops that become contaminated with viral particles when an infected person coughs, sneezes, or touches the surface. The transmission cycle is completed when a susceptible person touches the contaminated fomite, picking up the viable virus on their hands. The virus then enters the body when the person touches a mucosal surface, such as rubbing their eyes or touching their mouth or nose. The ability of a virus to survive on a surface varies widely, with some viruses remaining infectious for hours to days.
Vector-Borne and Ingestion Transmission
Some viruses rely on an intermediate living organism, called a vector, to move from one host to another. Vector-borne transmission most often involves arthropods, such as mosquitoes, ticks, or fleas, which become infected after feeding on an infected host. The virus reproduces within the vector and is transmitted to a new host through the vector’s bite, which is the mechanism for viruses like West Nile and Dengue. Another distinct route is ingestion transmission, commonly known as the fecal-oral route, which occurs when viral particles from an infected person’s stool are swallowed by another person. This pathway is often facilitated by poor sanitation or hygiene, where trace amounts of feces contaminate food, water sources, or hands, and even microscopic amounts of viral material can be sufficient to cause infection.