Disease is an abnormal state of the body that is associated with specific signs and symptoms. It represents a break down in the delicate physiological balance (homeostasis) maintained by control mechanisms whose precise workings are not fully understood.
Disease can be caused by microorganisms that invade the human body by overcoming or bypassing innate immune defense mechanisms and stably colonizing a niche. Once established, the pathogen can cause infectious disease by releasing toxins that damage host tissues. Pathogens differ greatly in their ability to achieve and maintain focus of infection, requiring an equally diverse set of defensive responses from the host.
An infectious disease is typically something that your physician diagnoses using a combination of your medical history, physical exam and a variety of laboratory tests. Infections can be caused by viruses, bacteria, fungi and protozoa. The word disease is also used more broadly to include a wide range of conditions that interfere with normal body functions, such as chronic diseases.
Some diseases are associated with specific social and economic standings, such as diseases of poverty or diseases of affluence. Others are linked to specific lifestyle factors such as poor nutrition, inactivity and excessive alcohol use, and may be referred to as lifestyle diseases.
Some diseases are inherited, meaning they can be passed on from generation to generation through genes. But a large proportion of diseases have no known hereditary basis and are said to be idiopathic, meaning they have an unknown cause. Research is ongoing in an effort to eradicate many of these diseases.