H

HAART:
See Highly Active Antiretroviral Therapy.

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

HCFA:
See Health Care Financing Administration.

HEALTH CARE FINANCING ADMINISTRATION (HCFA):
The federal agency that administers the Medicare, Medicaid, and Child Health Insurance Programs. Internet address: http://www.hcfa.gov/.

HEALTH RESOURCES AND SERVICES ADMINISTRATION (HRSA):
A U.S. Department of Health and Human Services agency that directs national health programs which improve the health of the Nation by assuring quality health care to underserved, vulnerable, and special-need populations and by promoting appropriate health professions workforce capacity and practice, particularly in primary care and public health. Among other functions, HRSA administers the Ryan White C.A.R.E. Act Titles I, II, III(b), IV, SPNS, and AETCs (see explanations for these terms under Ryan White C.A.R.E. Act) to provide treatment and services for those affected by HIV/AIDS. HRSA administers programs to demonstrate how communities can organize their health care resources to develop an integrated, comprehensive, culturally competent system to care for those with AIDS and HIV infection. HRSA also administers education and training programs for health care providers and community service workers who care for persons living with HIV or AIDS. Internet address: http://www.hrsa.dhhs.gov/.

HELPER/SUPPRESSOR RATIO (Of T cells):
T cells are lymphocytes (see leukocytes) that are formed in the thymus and are part of the immune system. They have been found to be abnormal in persons with AIDS. The normal ratio of helper T cells (see CD4+ cells) to suppressor T cells (see CD8+ cells) is approximately 2:1. This ratio becomes inverted in persons with AIDS but also may be abnormal for a host of other temporary reasons.

HELPER T CELLS:
Lymphocytes bearing the CD4 marker that are responsible for many immune system functions, including turning antibody production on and off.

HEMATOCRIT:
A laboratory measurement that determines the percentage of packed red blood cells in a given volume of blood. In women, red blood cells are normally 37 to 47 percent of their blood, and in men, red blood cells are normally 40 to 54 percent of their blood.

HEMATOTOXIC:
Poisonous to the blood or bone marrow.

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

HEMOLYSIS:
The rupture of red blood cells.

HEMOPHILIA:
An inherited disease that affects mostly males and prevents normal blood clotting. It is treated by lifelong injections of a synthetic version of the clotting factor lacking in persons with the disease. The new recombinant clotting factor replaces the natural product, which was extracted from people's blood and, when not heat treated, could carry HIV.

HEPATIC:
Pertaining to the liver.

HEPATITIS:
An inflammation of the liver. May be caused by bacterial or viral infection, parasitic infestation, alcohol, drugs, toxins, or transfusion of incompatible blood. 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: (1) hepatitis A, caused by infection with the hepatitis A virus, which is spread by fecal-oral contact; (2) 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 of drug needles; (3) non-A, non-B hepatitis, caused by the hepatitis C virus, which appears to be spread through sexual contact as well as through sharing of drug needles (another type of non-A, non-B hepatitis is caused by the hepatitis E virus, principally spread through contaminated water); (4) delta hepatitis, which occurs only in persons 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 persons with hemophilia.

HEPATOMEGALY:
Enlargement of the liver.

HERPES VIRUSES:
A group of viruses that includes herpes simplex type 1 (HSV-1), herpes simplex type 2 (HSV-2), cytomegalovirus (CMV), Epstein-Barr virus (EBV), varicella zoster virus (VZV), human herpes virus type 6 (HHV-6), and HHV-8, a herpes virus associated with Kaposi's Sarcoma.

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. Stress, trauma, other infections, or suppression of the immune system can reactivate the latent virus.

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.

HERPES VARICELLA ZOSTER VIRUS (VZV):
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.

HIGHLY ACTIVE ANTIRETROVIRAL THERAPY (HAART):
The name given to treatment regimens recommended by leading HIV experts to aggressively suppress viral replication and progress of HIV disease. The usual HAART regimen combines three or more different drugs such as two nucleoside reverse transcriptase inhibitors (see NRTIs) and a protease inhibitor, two NRTIs and a non-nucleoside reverse transcriptase inhibitor (see NNRTI) or other combinations. These treatment regimens have been shown to reduce the amount of virus so that it becomes undetectable in a patient's blood. For information about the U.S. Public Health Service recommendations for the treatment of HIV infections in adults, adolescents, children, and pregnant women, call the HIV/AIDS Treatment Information Service at 1-800-448-0440. Internet address: http://www.hivatis.org/.

HISTOCOMPATIBILITY TESTING:
A method of matching the self-antigens on the tissues of a transplant donor with those of a recipient. The closer the match, the better the chance that the transplant will not be rejected. See Human Leukocyte Antigens.

HISTOPLASMOSIS:
A fungal infection, commonly of the lungs, caused by the fungus Histoplasma capsulatum. This fungus is commonly found in bird and/or bat droppings in the Ohio and Mississippi Valley region, the Caribbean Islands, and in Central and South America. It is spread by breathing in the spores of the fungus. The most definitive test for the fungus has been from fungal stains and bone marrow cultures. Blood testing has proved to be less reliable. In areas where H. capsulatum is prevalent, 80 percent or more of the population has been exposed to infection through breathing in airborne spores produced by the fungus. Persons with severely damaged immune systems, such as those with AIDS, are vulnerable to a very serious disease known as progressive disseminated histoplasmosis. Nationwide, about 5 percent of persons with AIDS have histoplasmosis, but in geographic areas where the fungus is common, persons with AIDS are at high risk for disseminated histoplasmosis.

HIV-1:
See Human Immunodeficiency Virus Type 1.

HIV-2:
See Human Immunodeficiency Virus Type 2.

HIV DISEASE:
During the initial infection with HIV, when the virus comes in contact with the mucosal surface, and finds susceptible T cells, the first site at which there is truly massive production of the virus in lymphoid tissue. This leads to a burst of massive viremia with wide dissemination of the virus to lymphoid organs. The resulting immune response to suppress the virus is only partially successful and some virus escapes. Eventually, this results in high viral turnover that leads to destruction of the immune system. HIV disease is, therefore, characterized by a gradual deterioration of immune functions. During the course of infection, crucial immune cells, called CD4+ T cells, are disabled and killed, and their numbers progressively decline. See Acquired Immunodeficiency Syndrome; Human Immunodeficiency Virus Type 1.

HIV-RELATED TUBERCULOSIS:
See Tuberculosis.

HIV SET POINT:
The rate of virus replication that stabilizes and remains at a particular level in each individual after the period of primary infection.

HIV VIRAL LOAD:
See Viral Load Test.

HIVID:
See Zalcitabine.

HLA:
See Human Leukocyte Antigens.

HODGKIN's DISEASE:
A progressive malignant cancer of the lymphatic system. Symptoms include lymphadenopathy, wasting, weakness, fever, itching, night sweats, and anemia. Treatment includes radiation and chemotherapy. See Lymphoma.

HOLISTIC MEDICINE:
Healing traditions that promote the protection and restoration of health through theories reputedly based on the body's natural ability to heal itself and through manipulation of various ways body components affect each other and are influenced by the external environment.

HOMOLOGOUS:
Similar in appearance or structure, but not necessarily in function.

HORMONE:
An active chemical substance formed in one part of the body and carried in the blood to other parts of the body where it stimulates or suppresses cell and tissue activity. See Pituitary Gland.

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 antiretroviral drug in controlling HIV infection.

HPV:
See Human Papilloma Virus.

HRSA:
See Health Resources and Services Administration.

HTLV-I:
See Human T Cell Lymphotropic Virus Type I.

HTLV-II:
See Human T Cell Lymphotropic Virus Type II.

HUMAN GROWTH HORMONE (HGH):
A peptide hormone secreted by the anterior pituitary gland in the brain. HGH enhances tissue growth by stimulating protein formation. A recombinant (genetically engineered) HGH, called Serostim, has been approved by FDA as a treatment for AIDS wasting syndrome.

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. 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 factory. See Lentivirus; Retrovirus.

HUMAN IMMUNODEFICIENCY VIRUS TYPE 2 (HIV-2):
A virus closely related to HIV-1 that has also been found to cause AIDS. It was first isolated in West Africa. Although HIV-1 and HIV-2 are similar in their viral structure, modes of transmission, and resulting opportunistic infections, they have differed in their geographic patterns of infection.

HUMAN LEUKOCYTE ANTIGENS (HLA):
Marker molecules on cell surfaces that identify cells as "self" and prevent the immune system from attacking them.

HUMAN PAPILLOMA VIRUS (HPV):
The virus that causes genital warts and is linked to cervical dysplasia and cervical cancer. HPV affects more than 24 million Americans, and CDC estimates that there are at least 500,000 new cases each year. There is no specific cure for an HPV infection, but the warts can be removed or controlled by podophyllotoxin, the active ingredient in podophyllin. Interferon is used in the treatment of refractory or recurrent genital warts. Cryotherapy, laser treatment, or conventional surgery can remove the warts. The virus can be transmitted through sexual contact. HPV is a frequently seen infection in women with HIV/AIDS.

HUMAN T CELL LYMPHOTROPIC VIRUS TYPE I (HTLV-I):
HTLV-I and HTLV-II, like all retroviruses, are single-stranded RNA viruses containing a genome that replicates through a DNA intermediate or intermediary. This unique life cycle is made possible by the presence of a virally encoded enzyme, reverse transcriptase, which converts a single-stranded viral RNA into a double-stranded DNA provirus that can then be integrated into the host genome. HTLV-I has an affinity for T lymphocytes; it appears to be the causative agent of certain T cell leukemias, T cell lymphomas, and HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP).

HUMAN T CELL LYMPHOTROPIC VIRUS TYPE II (HTLV-II):
A virus closely related to HTLV-I, shares 60 percent genomic homology (structural similarity) with HTLV-I. Found predominantly in IV drug users and Native Americans, as well as Caribbean and South American Indian groups. HTLV-II has not been clearly been linked to any disease, but has been associated with several cases of myelopathy/tropical spastic paraparesis (HAM/TSP)-like neurological disease.

HUMORAL IMMUNITY:
The branch of the immune system that relies primarily on antibodies. See Cell-Mediated Immunity.

HYBRID:
An offspring produced from mating plants or animals from different species, varieties, or genotypes.

HYBRIDOMA:
A hybrid cell produced by the fusion of an antibody-producing lymphocyte with a tumor cell. Hybridomas are used in the production of monoclonal antibodies.

HYDROXYUREA:
An inexpensive prescription drug used for the treatment of sickle-cell anemia and some forms of leukemia. Recent treatment studies have shown that hydroxyurea can boost the antiretroviral effects of the anti-HIV drug ddI and possibly of other antiretroviral drugs in HIV-infected people. Since hydroxyurea affects a human host cells enzyme, rather than an HIV enzyme, there may be less chance for the development of resistant virus strains. Although viral load may be reduced significantly in patients on regimens containing hydroxyurea, increases of CD4 cells may be small because of hydroxyurea's suppressive effect on the bone marrow. This can cause neutropenia (presence of abnormally small numbers of white blood cells in the circulating blood) or lymphopenia.

HYPERGAMMAGLOBULINEMIA:
Abnormally high levels of immunoglobulins in the blood. Common in persons with HIV.

HYPERGLYCEMIA:
An abnormally high concentration of glucose in the circulating blood, seen especially in patients with diabetes mellitus.

HYPERPLASIA:
Abnormal increase in the elements composing a part (as tissue cells).

HYPERTHERMIA:
An unproven and dangerous experimental procedure that involves temporarily heating a patient's body core to temperatures of up to 108^o F on the theory that this temperature kills free HIV and HIV-containing cells. One method for accomplishing this is by passing patients blood through an external heater. This is called extracorporeal whole body hyperthermia.

HYPOGAMMAGLOBULINEMIA:
Abnormally low levels of immunoglobulins. See Antibodies.

HYPOTHESIS:
A specific statement or proposition, stated in a testable (researchable) form, predicting a particular relationship among multiple variables.

HYPOXIA:
Reduction of oxygen supply to tissues.