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).

Cases grouped together in one place and time, often before anyone knows whether the number is more than expected. CDC's glossary counts a cluster either way; its outbreak lesson describes one as suspected to be more than expected. A cluster is a reason to look closer; it may or may not turn out to be an outbreak.

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. It can be any health problem, not just an infection: tests talk about opioid-overdose and obesity epidemics too.

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.

The study of disease itself: its nature, causes, development and effects, often by examining tissues and body fluids in a lab.

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

An agent's ability to cause severe disease.

An agent's ability to infect: the share of exposed people who become infected. (Pathogenicity is the share of infected who get sick; virulence, the share of sick who become severely ill or die.)

An illness whose cause is unknown.

A pathogen's preference for certain cells, tissues or hosts: HIV infects CD4 immune cells, rabies travels along nerves, HPV infects skin and mucous membranes.

An infection with no noticeable symptoms. The person may still be able to spread it.

An infection by an organism that rarely sickens healthy people but does harm people with weak immune systems.

A lasting after-effect of a disease: rheumatic heart disease after strep throat, paralysis after polio.

A life-threatening, body-wide reaction to an infection that starts damaging the body's own organs.

Coming on suddenly and severely (fulminant liver failure can develop within days).

Any factor that affects the health of a population: biology, behavior, environment, or social conditions like income and education (the "social determinants of health").

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

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).

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

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

An infectious disease that can spread from one person or animal to another, directly or indirectly. (Tetanus is infectious but not communicable.)

Vertical: from a mother to her baby before, during or soon after birth (hepatitis B, rubella, Zika). Horizontal: from person to person in the population.

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

The animal versions of epidemic and endemic: an outbreak among animals (bird flu sweeping through poultry farms), or a disease always present in an animal population.

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.

A person or animal infected with an agent who can spread it while showing no signs of the disease (at least at that time: incubatory and convalescent carriers count).

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."

The case that brings a disease into a population: the true "patient zero."

A person who caught the disease from contact with a primary case.

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.

Immunity your own body builds by making antibodies, after an infection or a vaccine. Usually long-lasting.

Borrowed, temporary immunity from antibodies made by someone else: passed from mother to baby, or given as an antibody shot.

The umbrella term for any process that makes someone's immune system protected against an agent: by a vaccine, by an earlier infection, or by receiving antibodies.

Giving a vaccine (a killed, weakened or partial form of an agent, or instructions to make one of its parts) so the body builds its own immunity. Edward Jenner's cowpox vaccine (1796) was the first.

Deliberately introducing an agent into the body to produce immunity. Before vaccines, variolation inoculated people with material from real smallpox sores.

Efficacy: how well a vaccine prevents disease under ideal conditions, as in a controlled trial. Effectiveness: how well it works in real-world use, usually a little lower (missed doses, storage problems, different people).

Protection from some vaccines fades over the years. Pertussis (whooping cough) protection drops within a few years, so teens get a Tdap booster, and a school outbreak can hit students whose last shot was longest ago. Babies too young to finish their own shots depend on the people around them being protected.

A weakened live form of the agent (MMR, chickenpox, oral polio). Strong, long-lasting immunity, often from one or two doses. But it needs refrigeration (the "cold chain"), usually can't be given to people with weak immune systems or to pregnant people, and very rarely can turn virulent again (vaccine-derived polio).

A killed form of the agent (the polio shot, hepatitis A, most flu shots). Safer, since it can't cause infection, but immunity is weaker and usually needs boosters.

A vaccine made from an inactivated toxin, so the body learns to block the toxin instead of the germ (tetanus and diphtheria).

An ingredient added to a vaccine (such as an aluminum salt) to make the immune response stronger.

Active natural: from getting infected. Active artificial: from a vaccine. Passive natural: antibodies from mother to baby, across the placenta or in breast milk. Passive artificial: an injection of antibodies or antitoxin (like rabies immune globulin). Active is long-lasting; passive fades within weeks to months.

No vaccine is 100% effective; they may have caught it before immunity built up (about 2 weeks after the dose); their protection may have waned; they may have missed a dose in the series; or the strain may not be one the vaccine covers.

Any step taken to prevent a disease rather than treat it, such as antibiotics after an anthrax exposure or malaria pills before a trip.

How close a measurement is to the true value.

How close repeated measurements are to each other. A scale that always reads 2 kg too heavy is precise but not accurate. A bigger sample makes an estimate more precise.

Validity: does the test measure what it's supposed to measure? Reliability (repeatability): does it give the same result when repeated? A test can be reliable without being valid.

Random error is scatter from chance: it lowers precision and shrinks with a bigger sample. Systematic error (bias) pushes every result the same way: it lowers accuracy, and a bigger sample won't fix it.

CDC: Centers for Disease Control and Prevention (U.S.). WHO: World Health Organization (United Nations). NIH: National Institutes of Health (U.S. medical research).

Morbidity and Mortality Weekly Report: CDC's weekly publication of outbreak reports, surveillance data and recommendations.

Médecins Sans Frontières (Doctors Without Borders): an international medical aid organization that responds to epidemics and emergencies.

Health Insurance Portability and Accountability Act (1996): the U.S. law that protects the privacy of people's health information.

HIV: human immunodeficiency virus. SARS: severe acute respiratory syndrome. COVID-19: coronavirus disease 2019, caused by SARS-CoV-2.

NCHS: National Center for Health Statistics, the part of CDC that compiles U.S. vital statistics. NHANES: the National Health and Nutrition Examination Survey, an NCHS survey that combines interviews with physical exams and lab tests.

Council of State and Territorial Epidemiologists: works with CDC to decide which diseases are nationally notifiable and writes their standard surveillance case definitions.

Shiga toxin-producing E. coli, such as O157:H7: causes bloody diarrhea and sometimes kidney failure (HUS).

MMR: measles, mumps and rubella vaccine. Tdap and DTaP: tetanus, diphtheria and pertussis vaccines (DTaP for young children, Tdap for older children and adults).

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.

Data from records of births, deaths (including fetal deaths), marriages and divorces. States collect them; NCHS compiles them for the whole U.S.

Finding and following up everyone a case may have exposed, so they can be tested, treated, given prophylaxis or quarantined. Monitoring contacts means checking them daily for symptoms for one incubation period (21 days for Ebola), without necessarily restricting their movement.

An infection that is new in a population, or whose cases or range are quickly growing (COVID-19, Zika, West Nile virus in the U.S.).

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