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HAIRY LEUKOPLAKIA: A whitish, slightly raised lesion that appears on the side of the tongue. Thought to be related to Epstein-Barr virus infection, it was not observed before the HIV epidemic. See also Epstein-Barr Virus.

HALF-LIFE: The time required for half the amount of a drug to be eliminated from the body.

HELPER/SUPPRESSOR RATIO: (Of T cells). T cells are lymphocytes (white blood cells) that are formed in the thymus and are part of the immune system; they have been found to be abnormal in people with AIDS. The normal ratio of helper T cells (CD4+ cells) to suppressor T cells (CD8+ cells) is approximately 2:1. This becomes inverted in people with AIDS, but may be abnormal for a host of other temporary reasons. See also CD4 (T4) or CD4+ Cells; CD8 (T8) Cells; Lymphocyte; Thymus.

HELPER T CELLS: See CD4 (T4) or CD4+ Cells.

HEMATOCRIT: A laboratory measurement that determines the percentage of packed red blood cells in a given volume of blood.

HEMATOTOXIC: Poisonous to the blood or bone marrow.

HEMOGLOBIN: The component of red blood cells that carries oxygen.

HEMOPHILIA: An inherited disease that prevents the normal clotting of blood.

HEPATITIS: An inflammation of the liver caused by certain viruses and other factors such as alcohol abuse, some medications and trauma. Although many cases of hepatitis are not a serious threat to health, the disease can become chronic and can sometimes lead to liver failure and death. There are four major types of viral hepatitis: (a) hepatitis A, caused by infection with the hepatitis A virus; (b) hepatitis B, caused by infection with the hepatitis B virus (HBV), which is most commonly passed on to a partner during intercourse, especially during anal sex, as well as through sharing drug needles; (c) non-A, non-B hepatitis, caused by the hepatitis C virus, which appears to be spread through sexual contact as well as through sharing drug needles (another type of non-A, non-B hepatitis is caused by the hepatitis E virus, principally spread through contaminated water) (d) delta hepatitis occurs only in people who are already infected with HBV and is caused by the HDV virus; most cases of delta hepatitis occur among people who are frequently exposed to blood and blood products such as people with hemophilia. See also Hemophilia.

HERPES SIMPLEX VIRUS I (HSV-I): A virus that causes cold sores or fever blisters on the mouth or around the eyes, and can be transmitted to the genital region. The latent virus can be reactivated by stress, trauma, other infections or suppression of the immune system.

HERPES SIMPLEX VIRUS II (HSV-II): A virus causing painful sores of the anus or genitals that may lie dormant in nerve tissue. It can be reactivated to produce the symptoms. HSV-II may be transmitted to a neonate (newborn child) during birth from an infected mother, causing retardation and/or other serious complications. HSV-II is a precursor of cervical cancer. See also Cervical Cancer.

HERPES VARICELLA ZOSTER VIRUS: The varicella virus causes chicken pox in children and may reappear in adults as herpes zoster. Also called shingles, herpes zoster consists of very painful blisters on the skin that follow nerve pathways.

HIV-1: See Human Immunodeficiency Virus Type 1.

HIV-2: See Human Immunodeficiency Virus Type 2.

HIV DISEASE: Characterized by a gradual deterioration of immune function. During the course of infection, crucial immune cells called CD4+ T cells are disabled and killed, and their numbers progressively decline. CD4+ T cells play a crucial role in the immune response, signaling other cells in the immune system to perform their special functions. See also Acquired Immunodeficiency Syndrome; CD4 (T4) or CD4+ Cells; Human Immunodeficiency Virus Type 1.

HIV-RELATED TUBERCULOSIS: See Tuberculosis.

HOST: A plant or animal harboring another organism.

HOST FACTORS: The body's potent mechanisms for containing HIV, including immune system cells called CD8+ T cells, which may prove more effective than any antiretro-viral drug in controlling HIV infection. See also Antiretroviral Agents; CD8 (T8) Cells.

HUMAN IMMUNODEFICIENCY VIRUS TYPE 1 (HIV-1): 1. The retrovirus isolated and recognized as the etiologic (i.e., causing or contributing to the cause of a disease) agent of AIDS. HIV-1 is classified as a lentivirus in a subgroup of retroviruses. See also Lentivirus; Retrovirus. 2. Most viruses and all bacteria, plants and animals have genetic codes made up of DNA, which uses RNA to build specific proteins. The genetic material of a retrovirus such as HIV is the RNA itself. HIV inserts its own RNA into the host cell's DNA, preventing the host cell from carrying out its natural functions and turning it into an HIV virus factory. See also DNA; Ribonucleic Acid.

HUMAN IMMUNODEFICIENCY VIRUS TYPE 2 (HIV-2): A virus closely related to HIV-1 that has been found to cause immune suppression. Most commonin Africa.

HUMAN PAPILLOMA VIRUS (HPV): A virus that is the cause of warts of the hands and feet, as well as lesions of the mucous membranes of the oral, anal and genital cavities. More than 50 types of HPV have been identified, some of which are associated with cancerous and precancerous conditions. The virus can be transmitted through sexual contact and is a precursor to cancer of the cervix. There is no specific cure for an HPV infection, but the virus often can be controlled by podophyllin (medicine derived from the roots of the plant Podophyllum peltatum) or interferon, and the warts can be removed by cryosurgery, laser treatment or conventional surgery. See also Cervical Cancer; Condyloma.

HYPOTHESIS: A tentative statement or supposition that may then be tested through research.