An infectious disease is a clinically clear disease of humans or animals that damages or injures the host so as to harm host function, and results from the presence and movement of one or additional pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular parasites, and unusual proteins known as prions. Transmission of an infectious disease may arise through several pathways; including through contact with infected individuals, by water, food, airborne inhalation, or through vector-borne spread.

A contagious disease (also called a communicable disease) is an infectious disease that is competent of being transmitted from one person or species to one more. Contagious diseases are often increasing through direct contact with an individual, contact with the bodily fluids of infected persons, or with objects that the infected person has contaminated. The word infectivity describes the ability of an organism to enter, live and multiply in the host, while the infectiousness of a disease indicates the relative ease with which the disease is transmitted to additional hosts. An infection though, is not synonymous with an infectious disease, as an infection may not cause clinical symptoms or damage host function.

Diagnosis of infectious disease is virtually always initiated by medical history and physical examination. More detailed recognition techniques involve the culture of infectious agents remote from a patient. Culture allows classification of infectious organisms by examining their tiny features, by detecting the presence of substances produced by pathogens, and by openly identifying an organism by its genotype. Other techniques (such as X-rays, CAT scans, PET scans or NMR) are used to create images of internal abnormalities resulting from the intensification of an infectious agent. The images are obliging in detection of, for example, a bone abscess or a spongiform encephalopathy formed by a prion.