The amount, or rate, of illness in a population.

The rate of death in a population.

Of everyone who gets a disease, the percentage who die from it.

The number (or rate) of new cases of a disease that occur in a population during a set time period.

The number of existing cases (new + old) of a disease in a population at a given time.

A special kind of incidence used during outbreaks: the percentage of an exposed group that becomes ill.

Among susceptible contacts of a known case, the share who get sick within one incubation period. The primary case is left out of the math.

Deaths of babies under 28 days old per 1,000 live births.

Deaths of babies under 1 year old per 1,000 live births.

Stillbirths (fetal deaths at 28+ weeks) plus deaths in the first 7 days, per 1,000 births.

Deaths of mothers from pregnancy or childbirth causes, per 100,000 live births.

A disease that occurs occasionally and irregularly, with no predictable pattern.

The constant, expected, baseline level of a disease that is normally present in an area or population.

A consistently low level of disease, so few people build immunity.

Persistently high levels of disease, above the usual endemic baseline.

A high level of infection that starts early in life, so most children are infected and most adults are immune (malaria in parts of Africa).

A group of cases grouped in time and place that seem greater than expected: even if the "normal" number isn't precisely known.

An increase in cases above what's normally expected, usually in a limited geographic area.

The same idea as an outbreak: more cases than expected, but often used for a larger area or population.

An epidemic that has spread across several countries or continents, affecting a large number of people.

The "thing" that causes disease: a bacterium, virus, chemical, or other cause.

Single-celled eukaryotes, either free-living or parasitic: historically called "one-celled animals." Plasmodium (malaria) and Giardia are examples.

Any substance the body recognizes as foreign (like a toxin, or a marker on the surface of a pathogen or transplanted organ) that triggers the immune system to make antibodies against it.

The cause (or set of causes) of a disease.

An agent's ability to cause disease once it has infected someone.

An agent's ability to cause severe disease.

The person (or animal) that an agent infects.

The natural "home" where an agent normally lives, grows, and multiplies: a person, animal, or part of the environment.

A living carrier (usually an insect or animal) that transports an agent from one host to another (e.g., mosquitoes and malaria).

An inanimate object (a doorknob, a towel) that carries an agent from one person to another.

An infectious disease that can pass from animals to humans (rabies, Salmonella from pet turtles).

Illness or harm caused by medical treatment itself (a drug side effect, an infection from surgery).

An infection picked up in a hospital or other health-care setting.

An agent that causes birth defects when a fetus is exposed during pregnancy (Zika virus, rubella, alcohol, thalidomide).

A nonliving object or substance, like contaminated food or water, that carries an agent to a new host.

A person infected with an agent who can spread it, whether or not they show symptoms themselves.

The first case of a disease that comes to investigators' attention. It isn't always the first person who got sick (that's the primary case), so the index case isn't necessarily "patient zero."

Lacking the immunity or resistance needed to fight off an infection, and therefore at risk of becoming a case.

When enough of a population is immune to a disease that it can no longer spread easily: indirectly protecting even those who aren't immune.

The ongoing, systematic collection, analysis, and use of health data to guide public health decisions and action.

The probability, or chance, that a health event (like getting a disease) will happen.

Separating and restricting the movement of people who may have been exposed to a contagious disease, to see if they become sick.

Separating people who are already sick with a contagious disease from those who are healthy.

A disease that health-care providers and labs are legally required to report to public health authorities.