Disease is the abnormal state or functioning of all or part of an organism. In humans, diseases are categorized as acute,
or severe and short-term; chronic, or long-term; and recurrent, or periodic. Human
diseases are usually classified according to the cause or causes, which are countless, and are grouped as external or internal in nature.
Infectious diseases are caused by such external agents as bacteria, viruses, and parasitic worms, and are
transmitted by humans, animals, insects, or substances. For example, malaria is transmitted by an insect and colds and viral pneumonia from person to person. Diseases caused by parasites usually have a more complex method of transmission. Pork tapeworms and sheep liver fluke require one or more intermediate host organisms to complete their life cycle. Other external agents that cause diseases include such chemical and physical agents as radiation, which causes radiation injury and aplastic anemia; irritants, which cause such occupational diseases) as black lung; drugs; poisons; and injury.
Diseases may arise from internal causes. These include hereditary disorders, which are transmitted by the genes and chromosomes of one or both parents. Huntington disease is a dominant genetic disease and is carried by one gene; thus only 1 parent need be affected by the disease in order to pass it on. Recessive genetic diseases do not show up in children unless both parents are carrying a gene for that particular disease. Sex-linked disorders are carried on the X chromosome. Genetic diseases such as Down syndrome are caused by defects in the chromosomes themselves, such as the presence of an extra chromosome or the loss of part of a chromosome. Congenital diseases arise from abnormal development of an individual throughout pregnancy and birth or from maternal influence, for example, congenital syphilis.
Each organ system is subject to particular diseases. The circulatory system is subject to heart diseases such as valve damage from arteriosclerosis, which narrows the blood vessels. Blood diseases include leukemia. The musculoskeletal system can be weakened by many diseases, including osteogenesis imperfect, which is the presence of weak, brittle bones. Tumors, or abnormal growths, may affect any organ or organ system.
Body systems can also be affected by disease. The immune system, which forms antibodies against foreign agents such as bacteria, can, in some diseases, manufacture antibodies that attack the body itselfÑfor example, the rheumatoid disease arthritis. Degenerative diseases occur as the result of the natural aging processes.
The psychosomatic disorders are believed to be the result of emotional stress. Examples include peptic ulcers and some forms of colitis.