AIDS
Acquired Immunodeficiency Syndrome, the condition caused by HIV (Human Immunodeficiency Virus). In order to be classified as an AIDS case, an individual must be infected with HIV and have at least one of the AIDS-indicator conditions. Center for Disease Control and Prevention (CDC) created an AIDS case definition in 1983 for the purpose of statistical monitoring of the AIDS epidemic. This definition was revised in 1985, 1987 and again in 1993 as additional scientific information came to be known about HIV infection. A diagnosis of AIDS usually requires:
  • A CD4+ T-lymphocyte count under 200 cells/uL (or less than 14% of total lymphocytes) AND laboratory evidence of HIV infection;

OR

  • Presence of an AIDS-defining indicator disease and the absence of another reason for immune impairment.

Antibody
A substance produced by the body to counteract infectious agents.

Antigen
A foreign substance that is capable of stimulating immune response.

Antiretroviral Drugs
Refers to drugs or agents that affect the capability of retroviruses such as HIV to reproduce, prescribed for the treatment of retroviral infections such as HIV infection. The three common groups of antiretroviral agents for the treatment of HIV are: protease inhibitors, nucleoside analogs, and non-nucleoside reverse transcriptase inhibitors.

Association
The extent to which the occurrence of two or more characteristics are related. The relation may be causal or non-causal.

bDNA (branched DNA) assay:
Laboratory test for measuring the amount of virus in blood plasma.

CD4 T-lymphocytes
A specialized type of cell that coordinates the immune system and is the main target of HIV (also called 'helper T-cell).

Cohort
Groups of persons who share a common attribute such as birth in a particular year or residence in a particular area.

Counseling & Testing
In the content of HIV/AIDS surveillance, 'Testing' refers to testing of an individual for HIV infection. This service is generally accompanied by patient 'Counseling' before and after the test regarding the test, test results, interpretation of the test and other related issues. The Massachusetts Department of Public Health HIV/AIDS Bureau sponsors numerous Counseling and Testing programs, which have services available in "Anonymous" (the client is not identified or known to the counselor) as well as "Confidential" (the client is known to the counselor) settings.

Cumulative Incidence
The risk of developing a particular disease over a specified period of time.

ELISA (enzyme-linked immunoabsorbent assay) test or EIA (enzyme immunosorbent assay)
A blood test that detects antibodies to HIV. The ELISA is commonly used as the initial screening test for the presence of antibodies to HIV. The ELISA test does not detect the disease AIDS but only indicates if viral infection has occurred. A positive HIV ELISA test is confirmed by a second, more specific test, such as the HIV Western blot test.

Epidemic
An increase above the usual or expected rate of occurrence of particular events in a population.

Epidemiology
The study of the distribution and determinants of health-related events or disease in specified populations.

Exposure
Contact with or possession of a characteristic that is suspected to influence the risk of developing a particular disease.

HAART: (Highly Active Anti-Retroviral Therapy)
Combination of antiretroviral drugs that suppress HIV to immeasurable or near unmeasurable levels.

HIV
Human Immunodeficiency Virus, the virus that causes AIDS (Acquired Immunodeficiency Syndrome.

Immunosuppression or Immune Deficiency
A state of the body where the immune system defenses do not function normally, thus making a person susceptible to diseases that they would not ordinarily develop; this can be the result of illness or the administration of certain drugs.

Incubation Period
The time interval between contact with a risk factor and the first clinical evidence of the resulting illness.

Incidence:
The frequency of a disease occurrence during a given time period in a specified population (e.g. number of AIDS cases diagnosed in a year in Massachusetts).

Incidence Rate:
The rapidity with which new cases of a particular disease occur within a given population.

Mode of Transmission:
The means by which HIV is transmitted from one individual to another (also referred to as the 'transmission risk').

Morbidity: 
The occurrence of an illness, disease, or injury.

Co-morbidity:
The occurrence of more than one illness, disease, or infection at the same time ( e.g. AIDS and TB).

Mortality Rate
The rate with which persons within a given population die from a particular disease.

NASBA
A test used to measure viral load in persons infected with HIV.

Non-Progressors
In the context of HIV/AIDS, it generally refers to individuals who have been infected with HIV for a long period of time but have not developed AIDS.

No Identified Risk (NIR)
An HIV/AIDS case for which the information on risk/mode of HIV transmission is not available.

Opportunistic Infections
Those diseases, which are caused by agents, those are commonly present in our bodies or environment but cause disease only when there is a change from normal, healthy conditions - such as when the immune system becomes suppressed.

Partner Notification
Refers to the process of informing the partners (sexual- or needle sharing) of HIV or STD infected individuals about their possible exposure to infection. The Massachusetts Department of Public Health has 'Partner Notification' services available for individuals who wish to utilize them; these services are confidential and protect the privacy of all individuals.

PCR (polymerase chain reaction)
A laboratory technique used to measure viral load.

Perinatal Transmission:
Generally refers to the transmission of a characteristic or condition from a mother to the child during pregnancy or during childbirth. HIV infection can be transmitted perinatally from an HIV-infected mother to her child.

Prevalence
The proportion of persons in a given population who have a particular disease or condition at a particular time. Prevalence can be thought of as a snapshot of all existing cases at a specified time.

Prevalence Rate
The total number of all individuals who have a disease at a particular time divided by the population at risk of having the disease at that point in time.

Rate:
The rapidity at which a health event occurs as indicated by the number of cases per number of people. The rate of AIDS infection refers to the number of AIDS cases diagnosed in a defined population (population size) per 100,000.

Sample:
A subset of a population that is chosen for investigation.

Seroconversion
The development of antibodies to a particular antigen. When people develop antibodies to HIV, they "seroconvert" from HIV antibody-negative to antibody-positive.

Serosurveillance
Refers to a specialized tool of disease surveillance where data for a disease is collected through the results of tests conducted on blood samples drawn from a selected population.

Seroprevalence
Refers to the prevalence of a disease as calculated based upon the results of blood tests conducted in the target population.

Serostatus
Positive or negative results of a diagnostic test, such as an ELISA, for a specific antibody.

Surveillance
Ongoing observation of a population for rapid and accurate detection of changes in the occurrence of a disease or condition.

Variable
A population characteristic that can be measured in various categories.

Viral Load
Refers to the number or amount of virus in an infected individual. In the context of HIV, it is expressed as the number of HIV RNA copies/ml. It is currently being measured through three different HIV detection tests: PCR assay, bDNA assay and NASBA.

Virus
A microorganism composed of a piece of genetic material (RNA or DNA), surrounded by a protein coat. To replicate, a virus must infect a cell and direct the host cell's cellular machinery to produce new viruses.

Western Blot
A blood test used to detect HIV antibody. The Western blot is used to "confirm" the results of the ELISA test.

Window Period
The time period between the initial infection and the time when antibodies can be measured.

Year of AIDS Diagnosis
The year in which the AIDS diagnosis was made. The statistics released from the Massachusetts AIDS Surveillance Program analyze AIDS cases by the year of AIDS diagnosis, not the year in which a report is received.

Year of AIDS Report
The year in which an AIDS case is reported to the surveillance program.


This information is provided by the Bureau of Infectious Disease within the Department of Public Health.