Describing a disease or agent that can be transmitted to a host and cause infection, or a person who is capable of spreading a pathogen to others.
Explanation
When an organism is infectious it possesses the ability to invade susceptible hosts and replicate, leading to disease. Infectious diseases arise from bacteria, viruses, fungi, parasites or prions, and they differ from non infectious conditions such as genetic disorders or toxic exposures. An infectious agent’s capacity to spread depends on its reservoir, mode of transmission, infectious dose and the susceptibility of potential hosts. Transmission routes include direct contact, respiratory droplets, airborne particles, contaminated food or water, vectors like mosquitoes, and contaminated surfaces. Some infectious diseases are highly contagious and readily pass from person to person, while others require vectors or environmental exposure; for example, tetanus is infectious but not contagious because the causative bacterium does not spread directly between people. The term infectious also refers to the period during which an infected individual sheds enough pathogen to infect others. Public health interventions aim to reduce infectiousness through vaccination, antimicrobial therapy, isolation and sanitation.
Representative diseases
Measles is one of the most infectious human diseases; its virus spreads via respiratory droplets and airborne transmission, with a single case capable of generating many secondary cases in susceptible populations. Tuberculosis, caused by Mycobacterium tuberculosis, is infectious and transmitted by inhaling aerosols expelled by coughing, though prolonged exposure is often required. Human immunodeficiency virus (HIV) is infectious through sexual contact, blood transfusion and vertical transmission from mother to child. Tetanus, resulting from Clostridium tetani spores contaminating wounds, is infectious because the bacterium can infect and produce neurotoxin in a host, but it is not spread between individuals. West Nile virus infection is transmitted by mosquitoes; infected birds serve as reservoirs, highlighting an infectious disease that relies on a vector.
Recognising whether a disease is infectious, and understanding its modes of transmission, informs prevention strategies and helps control the spread of pathogens.
Related Terms: Infection, Contagious, Pathogen, Transmission, Epidemiology