A disease is a state of illness that is caused by a disruption to the normal homeostatic and defensive controls of the body. It occurs when a harmful influence overtakes these controls, and may take many forms, including infection by viruses, bacteria, parasites, and certain chemical agents like lead or toxins.
Diseases are usually characterized by signs and symptoms. A set of characteristic signs and symptoms that may be associated with a disease is called a syndrome. Signs and symptoms are diagnostic because they allow physicians and nurses to identify a disease and help determine its cause.
Once a disease is diagnosed, a treatment plan can be developed. This may include medications, surgery, diet, lifestyle changes, or other interventions. The goal is to restore the health state, if possible. A cure is a treatment that completely relieves the signs and symptoms of a disease.
Research on diseases is important to ensure the health of humankind. There is much work to be done, however, since even though great strides have been made in preventing diseases and curing them, some are still incurable.
It is also necessary to keep in mind the social impact of disease. It has been shown that disease can have profound societal consequences, including ostracism and discrimination. This can disrupt disease eradication efforts, cultivate distrust of the health authorities, and even result in civil unrest and violence. Diseases can also be transmissible from person to person, for example by hepatitis B and HIV.