measles: Highly contagious skin disease caused by a virus in family Paramyxoviridae. The virus enters the body through the respiratory tract or the conjunctiva. Measles is endemic throughout the world.

microbiota (microbial flora): Microorganisms that are normally associated with a particular tissue or organ.

morbidity rate: Number of individuals who become ill with a particular disease within a susceptible population during a specified time period.

mortality rate: Ratio of the number of deaths from a particular disease to the total number of cases of the disease.

nonspecific immunity: General defense mechanisms that provide animals with protection from infection and disease but are not targeted at a particular pathogen.

nosocomial infection: Infection produced by a pathogenic agent that a patient acquires during hospitalization or treatment inside another health care facility.

opportunistic organism: Organism that is usually harmless, but can be pathogenic in a compromised host.

pandemic: Increase in the occurrence of a disease in a large and geographically widespread population. Sometimes called a worldwide epidemic.

parasite: Organism that lives on or within another organism (the host). The relationship benefits the parasite and harms the host.

pasteurization: Process of heating milk and other liquids to destroy microorganisms that can cause spoiling or disease.

pathogen: Disease-producing agent.

pathogenicity: Ability to cause disease.

penicillins: Group of antibiotics that are often used to treat infections by gram-positive bacteria.

peptidoglycan: Large polymer that provides much of the strength and rigidity of bacterial cell walls.

period of infectivity: Time during which the source of an infectious agent is disseminating the agent (is infectious).

plague: Acute, infectious disease with a high mortality rate; caused by Yersinia pestis.

plasmid: Circular, double-stranded DNA molecule that can exist and replicate independently of the host cell chromosome or be integrated with it. Although a plasmid is stably inherited, it is not required for bacterial cell growth and reproduction.

poliomyelitis: Acute, contagious viral disease of the central nervous system that can lead to paralysis.

population: Group of organisms of the same species.

prevalence rate: Total number of people infected at one time in a population, regardless of when the disease began.

prion: Infectious particle that is responsible for certain slow-acting diseases such as scrapie in sheep and goats, and Creutzfeldt-Jakob disease in humans. Prions have a protein component, but scientists have not yet detected a nucleic acid component.

prokaryotic cell: Cell that lacks a membrane-delimited nucleus and other membrane-bound organelles. Bacteria are prokaryotic cells.

rabies: Acute infectious disease of the central nervous system caused by an RNA virus of the rhabdovirus group.

reservoir: Site, alternate host, or carrier that harbors pathogenic organisms and serves as a source from which other individuals can be infected.

retrovirus: RNA virus that carries the enzyme reverse transcriptase and forms a DNA copy of its genome during its reproductive cycle.

schistosomiasis: Helminth infection acquired from contact with water containing infected snails.

smallpox: Highly contagious, often fatal disease caused by a poxvirus. Smallpox has been eradicated throughout the world.