What is disease? In human beings, "disease" is often used more broadly to refer to any condition that causes extreme pain , dysfunction , distress , social problems , and/or death to the person afflicted, or similar problems for those in contact with the person. In this broader sense, it sometimes includes injuries , disabilities , disorders , syndromes , infections , isolated symptoms , deviant behaviors , and atypical variations of structure and function, while in other contexts and for other purposes these may be considered distinguishable categories.

This term broadly refers to any abnormal condition that impairs normal function. Commonly, this term is used to refer specifically to infectious diseases, which are clinically evident diseases that result from the presence of pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular parasites, and aberrant proteins known as prions. An infection that does not produce clinically evident impairment of normal functioning is not considered a disease. Non-infectious diseases are all other diseases, including most forms of cancer, heart disease, and genetic disease.