Disease is any harmful deviation from the normal structural and functional state of an organism usually associated with certain signs and symptoms. Diseases are often categorized as hereditary (genetic), drug-induced, autoimmune, infectious, communicable and degenerative. Diseases may also be distinguished by how they are transmitted, such as through hand-to-mouth contact or the consumption of contaminated water or food.
Diseases are caused by microorganisms, such as bacteria, viruses, protozoa, and fungi. These organisms are capable of penetrating the body’s innate defense mechanisms, colonizing the host, and reproducing within the host cell, which leads to infection or the production of toxic substances. These substances are released into the extracellular fluid and may infect adjacent cells or spread to distant sites via lymphatics or bloodstream.
A disease is typically identified by the presence of one or more signs, such as a fever, rash, or swollen ankle. Signs can be observed by others, such as physicians and nurses, or they may occur internally, such as unsteadiness or forgetfulness. A diagnosis is made by healthcare professionals, who evaluate a patient’s health status using a combination of medical history, physical examination and laboratory tests.