Finding New Viruses

Viruses are smaller and less complex than cells, and generally contain only a protein coat surrounding either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) as the genetic material. Some viruses may also have a covering membrane derived from the plasma membrane of the host cell from which they budded out during replication. DNA viruses include adenoviruses (respiratory infections); herpesviruses (cold sores, genital herpes, "pox" diseases, mononucleosis); papillomaviruses (certain warts); and hepatitis B virus. Viruses with genetic information in RNA include hepatitis A and C viruses; myxoviruses (measles, mumps, influenzas); picorna-

viruses (polio, respiratory infections); rabies virus; and retroviruses (AIDS, some leukemias). Viruses that infect animals without causing disease may jump species to infect humans or mutate to produce major disease epidemics, such as influenzas from recombinant bird or pig viruses, or the AIDS and Ebola viruses. In some cases, the animal source or reservoir is known, while often it is unknown. Many emerging diseases in human populations have originated in animals native to rain forest jungles or other habitats that humans have invaded and taken over, encountering previously unknown viruses.

Malignant tumors contain many more abnormal cells, which can leave the primary tumor site and form secondary tumors, especially in the liver, bone marrow, lungs, or brain, that usually cause death in cancer conditions. Malignant cells lose their original characteristics and become much less specialized and less efficient in using nutrients and energy, causing the body to waste away.

Degenerative diseases are associated with the aging process, when body tissues lose the ability to repair themselves effectively. The immune system also becomes less functional in combating foreign antigens or even recognizing the difference between self and foreign antigens, thus attacking self antigens by mistake. In some cases, degenerative diseases are not directly linked to aging, but to damage caused by pathogens, toxins, nutrient deficiency, or even nutrient excess. As normal tis sues are damaged by the standard wear and tear of life, repairs become less effective and scar tissue replaces normal tissues such as muscle or liver. Nervous system damage is particularly problematic, since neurons are unable to replicate in mature animals, and lost cells are not replaced, producing sensory reception, muscle control, and memory loss. Problems caused by diseases and trauma lead to continued loss of function over time in aging animals, eventually reaching a point that repairs can no longer be made or infections resisted, and the animal dies.

—Jean S. Helgeson See also: Aging; Biology; Cell types; Death and dying; Ecosystems; Food chains and food webs; Genetics; Immune system; Life spans; Mutations; Nutrient requirements; Pollution effects; Protozoa; Symbiosis; Veterinary medicine.

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