Disease is an illness or a condition that impairs the normal function of an organism or its cells. It may be caused by microorganisms, such as bacteria, viruses, protozoa, or fungi, or multicellular organisms such as cancer cells or aberrant proteins known as prions. It may also be caused by damage to tissues or organs of the body or by environmental factors, such as toxic chemicals or radiation. Diseases can be cured by medicines, treatments or procedures, and by lifestyle interventions.

The normal state of an organism consists of a delicate physiological balance, or homeostasis, among chemical, physical, and functional processes that are maintained by a complex set of mechanisms, many of which are not fully understood. Diseases are a departure from this normal state, and they often present signs and symptoms that highlight their abnormality.

Some diseases are contagious, and they can spread from one person to another through hand-to-hand contact or contaminated water or food. They can also be transmitted from animals to people or to other plants, or they may occur as a result of air pollution or radioactive contamination.

A disease can also be genetic, arising from a variation in an individual’s DNA that makes him or her more susceptible to certain environmental influences. Some genetic diseases are also inherited from the parents, but others can occur spontaneously in some people.