Infectious disease

An illness caused by a pathogen that enters or multiplies in a host. It may spread between hosts or arise from exposure to an environmental source.

Pathogen: A biological agent that causes disease in a host, including certain bacteria, viruses, fungi, and parasites. Infectious diseases arise when these agents establish themselves in a host.
Bacteria: Single-celled prokaryotic organisms, some of which cause disease in humans and other hosts. Bacteria are one major group of infectious agents.
Epidemiology: The study of the distribution and determinants of health-related conditions in populations. It tracks outbreaks and identifies factors that shape infectious disease spread.
Tuberculosis: An infectious disease caused by Mycobacterium tuberculosis, usually affecting the lungs. It illustrates a bacterial infection that can persist and spread through the air.
Noncommunicable disease: A disease that is not transmitted from one host to another and is generally not caused by an infectious agent. It contrasts with diseases whose causes are pathogens, though an infectious disease need not spread between people.
Infection: The invasion and multiplication of a pathogen in the body or another host. Infection is the pathogen's establishment; disease occurs when infection disrupts health.
Virus: An infectious agent that replicates inside living cells using host cellular machinery. Viruses cause infections but require host cells to reproduce.
Infectious disease diagnostics: Methods used to detect pathogens or evidence of infection in clinical specimens. Diagnosis identifies the cause of illness and can guide treatment and control.
Influenza: A contagious respiratory illness caused by influenza viruses. Its recurring epidemics show how viral change affects population disease.
Autoimmune disease: A disease in which the immune system mistakenly attacks the body's own tissues. Its cause is immune misdirection rather than a pathogen, despite possible symptom overlap.

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