According to the WHO, a disease is “any harmful deviation from an organism’s normal structural or functional state, generally associated with certain signs and symptoms and differing in nature from physical injury.”
A disease is referred to as a disorder of structure or function. It harms the organism it occurs within and has specific signs or symptoms.
Diseases can broadly be classified into communicable (infectious) and non-communicable (noninfectious) diseases. Influenza, common cold, typhoid, and pneumonia are all diseases.