In developed countries, the diseases that cause the most sickness overall are neuropsychiatric conditions, such as depression and anxiety. The study of disease is called pathology, which includes the study of etiology, or cause. Terminology
Concepts
In many cases, terms such as disease, disorder, morbidity, sickness and illness are used interchangeably; however, there are situations when specific terms are considered preferable. Disease
The term disease broadly refers to any condition that impairs the normal functioning of the body. For this reason, diseases are associated with the dysfunction of the bodys normal homeostatic processes. Commonly, the 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 organisms, and aberrant proteins known as prions. An infection or colonization that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal bacteria and yeasts in the gut, or of a passenger virus, is not considered a disease. By contrast, an infection that is asymptomatic during its incubation period, but expected to produce symptoms later, is usually considered a disease. Non-infectious diseases are all other diseases, including most forms of cancer, heart disease, and genetic disease. Acquired disease
An acquired disease is one that began at some point during ones lifetime, as opposed to disease that was already present at birth, which is congenital disease. Acquired sounds like it could mean "caught via contagion", but it simply means acquired sometime after birth.
Examples of the first-mentioned type are that Turner syndrome and DiGeorge syndrome are still often called by the "syndrome" name despite that they can also be viewed as disease entities and not solely as sets of signs and symptoms. Predisease
Predisease is a subclinical or prodromal vanguard of a disease. Prediabetes and prehypertension are common examples. The nosology or epistemology of predisease is contentious, though, because there is seldom a bright line differentiating a legitimate concern for subclinical/prodromal/premonitory status (on one hand) and conflict of interest–driven disease mongering or medicalization (on the other hand). Identifying legitimate predisease can result in useful preventive measures, such as motivating the person to get a healthy amount of physical exercise, but labeling a healthy person with an unfounded notion of predisease can result in overtreatment, such as taking drugs that only help people with severe disease or paying for drug prescription instances whose benefit–cost ratio is minuscule (placing it in the waste category of CMS "waste, fraud, and abuse" classification). Three requirements for the legitimacy of calling a condition a predisease are:
a truly high risk for progression to disease – for example, a pre-cancer will almost certainly turn into cancer over time
actionability for risk reduction – for example, removal of the precancerous tissue prevents it from turning into a potentially deadly cancer
benefit that outweighs the harm of any interventions taken – removing the precancerous tissue prevents cancer, and thus prevents a potential death from cancer.