virus A microorganism that is smaller than a bacterium that cannot grow or reproduce apart from a living cell. A virus invades living cells and uses their chemical machinery to keep itself alive and to replicate itself. It may reproduce with fidelity or with errors (mutations); this ability to mutate is responsible for the ability…
virus, attenuated A virus that has been weakened. A vaccine against a viral disease can be made from an attenuated, less virulent strain of the virus: a virus that is capable of stimulating an immune response and creating immunity but not of causing illness.
visceral leishmaniasis See leishmaniasis.
visceral pericardium The inner layer of the pericardium.
vision, macular See central vision.
vision therapy The use of special eye exercises to address eye defects, such as strabismus. Some vision therapists claim that eye exercises can help people with neurological or learning disabilities. Vision therapy is not proven for the latter use, although some patients do report improvement.
visual acuity The clarity or clearness of vision, a measure of how well a person sees.
visual acuity test See eye chart test.
visual field test A test that measures the extent and distribution of the field of vision. A visual field test may be done via a number of methods, including termed confrontation, tangent screen exam, and automated perimetry. These tests are not painful or uncomfortable. Many diseases can adversely affect the visual field, including glaucoma, strokes,…
visual nerve See optic nerve.
visual nerve pathways See optic nerve pathways.
vital Necessary to maintain life. For example, breathing is a vital function.