Disease occurs when some influence upsets the delicate physiological balance (homeostasis) of an organism. This imbalance may result from external sources like bacteria or viruses, or internal ones such as autoimmune dysfunction or genetic dysregulation. The result is often chronic, and the underlying processes are not always fully understood.
The word “disease” is often thrown around indiscriminately and sometimes in misleading ways, including by dictionaries that lump together conditions like cancer, high blood pressure, and depression as diseases despite the fact they are very different entities. These broader definitions may also obscure the important distinction between a disease and a health problem, such as feelings of pain, discomfort or distress.
At the most basic level, disease arises when a pathogen colonizes the mammalian body and establishes a focus of infection by overcoming or bypassing the innate defense mechanisms of the epithelia and tissues that line the respiratory, digestive and genital tracts. The process of pathogenesis can be complex and variable, as shown by the wide range of infectious diseases that affect mammals.
The term is used to describe all sorts of things, ranging from infections caused by bacteria and viruses to genetic diseases and disorders that are passed on through the family tree. The World Health Organization has catalogued all known diseases in a global classification system called the International Classification of Diseases, which is updated periodically. Each disease entity has a unique constellation of signs and symptoms that are diagnostic indications, but some illnesses such as dehydration have similar sets of signs and are therefore classified as syndromes rather than individual diseases.