Infectious refers to the ability of a microorganism to cause an infection and, in many cases, to be transmitted from one host to another.
Explanation
An infectious agent is any bacterium, virus, fungus, parasite or prion that can invade a susceptible host and replicate. Diseases resulting from such agents are known as infectious diseases. Infectiousness describes how readily an organism is transmitted between hosts; it depends on factors such as the organism’s mode of transmission, survival outside the host and the infectious dose needed to establish infection. Airborne viruses like measles and influenza are highly infectious because they spread easily via respiratory droplets and can initiate infection with few particles. By contrast, environmental pathogens such as Clostridium tetani cause infection after spores are introduced into deep wounds but do not spread from person to person.
Public health terminology distinguishes infectious diseases from non‑infectious conditions such as genetic disorders or degenerative diseases. Within infectious diseases, “communicable” or “contagious” describes those that are directly transmitted between individuals, whereas “infectious” encompasses agents acquired from environmental sources, vectors or animals. The period during which an infected host can transmit an agent is the infectious period, which may precede symptoms. Infectivity, pathogenicity and virulence are related but distinct concepts that describe the ease of infection, the ability to cause disease and the severity of disease. Control measures such as vaccination, sanitation, vector control and safe food handling reduce the spread of infectious agents.
Examples
- Measles virus is one of the most infectious pathogens known; a single case can infect dozens of susceptible individuals in an unvaccinated community.
- Influenza viruses are moderately infectious and spread through aerosols and direct contact, causing seasonal epidemics.
- Hepatitis B virus is infectious and transmissible through blood and bodily fluids but requires close contact or needle sharing.
- Legionella pneumophila causes an infectious pneumonia acquired from inhaling contaminated aerosolized water but is not transmitted person to person.
- Clostridium tetani spores are infectious when introduced into wounds but tetanus is not contagious.
The term “infectious” helps classify diseases and informs strategies to prevent transmission. Understanding how infectious agents spread guides public health interventions and personal protective behaviors.
Related Terms: communicable, contagious, infection, transmissibility, pathogen