Thus diagnostic terms often only reflect a symptom or set of symptoms (syndrome). Classical classification of human disease derives from the observational correlation between pathological analysis and clinical syndromes. Today it is preferred to classify them by their cause if it is known.The most known and used classification of diseases is the World Health Organizations ICD. This is periodically updated. Currently, the last publication is the ICD-11. Causes
Only some diseases such as influenza are contagious and commonly believed infectious. The microorganisms that cause these diseases are known as pathogens and include varieties of bacteria, viruses, protozoa, and fungi. Infectious diseases can be transmitted, e.g. by hand-to-mouth contact with infectious material on surfaces, by bites of insects or other carriers of the disease, and from contaminated water or food (often via fecal contamination), etc. Also, there are sexually transmitted diseases. In some cases, microorganisms that are not readily spread from person to person play a role, while other diseases can be prevented or ameliorated with appropriate nutrition or other lifestyle changes. Some diseases, such as most (but not all) forms of cancer, heart disease, and mental disorders, are non-infectious diseases. Many non-infectious diseases have a partly or completely genetic basis (see genetic disorder) and may thus be transmitted from one generation to another. Social determinants of health are the social conditions in which people live that determine their health. Illnesses are generally related to social, economic, political, and environmental circumstances.
YPLL measurements do not account for how disabled a person is before dying, so the measurement treats a person who dies suddenly and a person who died at the same age after decades of illness as equivalent. In 2004, the World Health Organization calculated that 932 million years of potential life were lost to premature death.The quality-adjusted life year (QALY) and disability-adjusted life year (DALY) metrics are similar but take into account whether the person was healthy after diagnosis. In addition to the number of years lost due to premature death, these measurements add part of the years lost to being sick. Unlike YPLL, these measurements show the burden imposed on people who are very sick, but who live a normal lifespan. A disease that has high morbidity, but low mortality, has a high DALY and a low YPLL. In 2004, the World Health Organization calculated that 1.5 billion disability-adjusted life years were lost to disease and injury. In the developed world, heart disease and stroke cause the most loss of life, but neuropsychiatric conditions like major depressive disorder cause the most years lost to being sick. Society and culture
How a society responds to diseases is the subject of medical sociology. A condition may be considered a disease in some cultures or eras but not in others. For example, obesity can represent wealth and abundance, and is a status symbol in famine-prone areas and some places hard-hit by HIV/AIDS.