Disease is a condition that causes signs or symptoms and interferes with normal functions of the body. The condition can be a result of an infection or from physical injury, but it also may be the result of some type of psychological stress. A disease is a medically defined abnormality that can be diagnosed by the use of diagnostic tests or by observing certain signs and symptoms.
The search for the causes (etiologies) of diseases dates back to antiquity, when Hippocrates and other Greek physicians realized that a patient’s illness is not a visitation from the gods but is caused by earthly influences. A disease that has no known cause is called idiopathic, although as medical science advances it may become possible to link some diseases with particular germs or other factors.
Many infectious diseases occur when microorganisms, such as bacteria, viruses, protozoa and fungi, enter the body and multiply. Infection results when a pathogen successfully penetrates the anatomic and physiological barriers that protect the body’s surface and stably colonizes a niche within the host cellular environment. Pathogens can also cause disease by releasing toxins, for example, bacterial toxins that kill host cells.
Some diseases are related to social standing and economic conditions, for example, poverty is associated with some diseases and affluence with others. Affluence is associated with long lifespans and sedentary lifestyles, which in turn are linked to a number of chronic diseases that are sometimes called “lifestyle diseases.” Some diseases can be spread from person to person or animal to person through direct contact. Other diseases can be spread by water, food and air, for example through pollution.