HIV defines a chronic RNA virus infection in the host. When opportunistic or other unusual or persistent infections have occurred or when the CD4+ lymphocyte count is less than 200 cells in persons older than age 12 years, a patient is defined as having AIDS. Opportunistic infections are caused by organisms in the environment that usually do not cause harm to persons who have intact immune systems, but can be the basis of serious or even fatal infection in those who have compromised immune systems. HIV enzyme-linked immunosorbent assay (ELISA) is a test performed on serum to detect antibodies against HIV antigens. If the ELISA results are positive, a Western blot is performed to identify individual antibodies to HIV antigens. A patient may have a positive ELISA result, but a negative Western blot result: This patient does not have HIV infection.