allergy: an unusually high sensitivity to a situation or substance, such as a food, pollen, or disease microorganism. Common symptoms of allergies include sneezing, itchiness, and skin rash.

ancestor: a relative, usually more remote than a grandparent, from whom a person is directly descended

antibiotic: a chemical substance produced by various molds and microorganisms that kills or weakens bacteria and some other microorganisms. Penicillin and streptomycin are examples of antibiotics that are used as medicines.

bacterium: a microscopic, single-celled organism that lives in colonies on or in the soil, water, or other organisms, including people; some species of bacterium cause disease. One bacterium; two or more bacteria.

cell: the smallest structural and functional unit of living organisms. Some cells can live as independent units, while others form colonies and are building blocks of more complicated organisms, such as plants and animals.

communicable disease: an infectious disease that can pass from an infected individual by direct or indirect contact to an unaffected individual through bodily fluids or by contact with an animal or insect carrier

contagion: the transmission or spreading of a disease by close contact

contagious disease: an infectious disease easily passed on to unaffected people by direct or indirect contact with infected individuals, their bodily fluids, or objects in the environment that they have contaminated

endemic: a word used to describe a contagious disease or condition that is present in a limited area and infecting a relatively small number of people

epidemic: the outbreak of a contagious disease, spreading from person to person, rapidly and widely, in an area where the disease is not otherwise common

immunity: the ability to resist a particular toxin or disease. People can be born with natural immunity to a disease, can develop immunity to future infection by surviving a bout of the disease, or can build up immunity by vaccination against the disease.

infection: an invasion of the body by a bacterium, virus, protozoan, or other disease-causing agent—and the body’s reaction to it

infectious disease: a disease caused by the invasion of a microorganism, such as a fungus, bacterium, or virus, that enters an organism. Infectious diseases spread from person to person or from the environment to a person.

inoculation: the introduction of a substance to the body to increase immunity to a disease

miasma: an unhealthy or unpleasant smell or vapor, such as a smell rising from a swamp or decaying matter. It was once believed that miasma was poisonous.

microbe: a microscopic organism, especially a disease-causing bacterium, virus, or protozoan. Microbes are best seen under a microscope.

microscopic: a word used to describe something that is too small to be seen by the unaided eye but is big enough to be seen with the help of a microscope

organism: a form of life, such as a plant, animal, bacterium, protozoan, mold, or virus, that is composed of interdependent parts that maintain various vital processes

pandemic: an epidemic that spreads across a wide area, covering countries, continents, and even the whole world

parasite: an organism that lives on or in another organism of a different species and that gets its nutrition from the other organism’s body

pathogen: a bacterium, virus, protozoan, or other microbe that causes disease—also called a germ pestilence: a usually fatal epidemic disease, such as bubonic plague

plague: a highly infectious, often fatal, epidemic disease. This term is sometimes used to refer to the bubonic plague.

pox: a disease, such as smallpox, that is characterized by skin eruptions. Sometimes the diseases smallpox and syphilis were nicknamed “the pox.”

protozoan: a one-celled organism, usually microscopic, that has the ability to move in simple ways and/or carry disease

pustule: a small, inflamed raised eruption or rash on the skin that is filled with pus

resistance: the natural ability of an organism, such as a human being, to fight off or resist a microbe or poison. Also, the ability of disease microbes to withstand or resist the effects of a drug that once was lethal to the microbes.

symbiotic: a close, long-term association between two or more living organisms of different species that is usually mutually beneficial

vaccination: inoculation with a harmless form of a disease to stimulate immunity against that disease. Vaccinations may be given by needle, inhalation, or mouth.

vector: a carrier or transporter of disease, often an insect or mite. The rat flea is the vector whose bite carries bubonic plague from rats to people.

virus: an ultramicroscopic infectious agent. Viruses are not usually considered life forms because they will only grow and reproduce within the cells of a living host.