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Frequently Asked Questions (FAQs) About HIV and AIDS
(FAQ Home)

The Human Immunodeficiency Virus (HIV) and Acquired Immune Deficiency Sydrome (AIDS)

How did AIDS get its name? Return to top

In 1982, CDC officially adopted the name, acquired immunodeficiency syndrome (AIDS), after meeting with groups composed of persons with hemophilia, blood industry officials, gay community organizations, and assorted leaders from CDC, the National Institutes of Health and the Food and Drug Administration. The September 24, 1982 Morbidity and Mortality Weekly Report (MMWR) defined a case of AIDS as "a disease at least moderately predictive of a defect in cell-mediated immunity occurring in a person with no known cause for diminished resistance to that disease."

What is AIDS? What causes AIDS? Return to top

AIDS stands for acquired immunodeficiency syndrome. An HIV-infected person receives a diagnosis of AIDS after developing one of the CDC-defined AIDS indicator illnesses. An HIV-positive person who has not had any serious illnesses also can receive an AIDS diagnosis on the basis of certain blood tests (CD4+ counts).

A positive HIV test result does not mean that a person has AIDS. A diagnosis of AIDS is made by a physician using certain clinical criteria (e.g., AIDS indicator illnesses).

Infection with HIV can weaken the immune system to the point that it has difficulty fighting off certain infections. These types of infections are known as "opportunistic" infections because they take the opportunity a weakened immune system gives to cause illness.

Many of the infections that cause problems or may be life-threatening for people with AIDS are usually controlled by a healthy immune system. The immune system of a person with AIDS is weakened to the point that medical intervention may be necessary to prevent or treat serious illness.

Today there are medical treatments that can slow down the rate at which HIV weakens the immune system. There are other treatments that can prevent or cure some of the illnesses associated with AIDS. As with other diseases, early detection offers more options for treatment and preventative care.

What is the evidence that HIV causes AIDS? Return to top

Although the scientific evidence is overwhelming and compelling that HIV is the cause of AIDS, the disease process is not yet completely understood.

This incomplete understanding has led some to make statements that AIDS is not caused by an infectious agent. This is not only misleading, but may have dangerous consequences. Before the discovery of HIV, evidence from epidemiologic studies involving tracing of patients' sex partners and cases occurring in blood recipients had clearly indicated that the underlying cause of the condition was an infectious agent. Infection with HIV has been the sole common factor shared by AIDS cases throughout the world among homosexual men, transfusion recipients, persons with hemophilia, sex partners of infected persons, children born to infected women, and occupationally exposed health care workers. Recommendations to prevent HIV and AIDS involve guidance to avoid or modify behaviors that pose a risk of transmitting the virus as well as the use of the HIV antibody test to screen donors of blood and organs.

Although HIV is the underlying cause of AIDS, much remains to be known about exactly how HIV causes immune deficiency. However, this incomplete understanding does not indicate that the virus is harmless. Why some persons exposed to HIV will become infected while others do not is also not known, but this is likely related to the amount of virus in the exposure and the route of entry (e.g., more than 90% of persons transfused with an HIV-infected unit of blood become infected).

Approximately half of HIV-infected adults will develop AIDS within 8 to 10 years of infection with HIV, while others continue to progress to AIDS after that period. As with any disease with a long latency period, it is possible that co-factors play a role in disease development. However, extensive epidemiologic and laboratory studies of HIV-infected persons have failed to identify any consistent factor, including drug use, malnutrition, or co-infections with other organisms, that affects the rate of progression to AIDS.

Clearly, more research is needed on the factors that contribute to HIV infection and the development of AIDS. However, the inescapable conclusions of more than 10 years of epidemiologic and virologic research are that most people, if exposed through sexual contact or injecting drug use, are susceptible to HIV infection, and if they become infected, most, if not all, persons will develop AIDS.

Below are citations to several articles by researchers at the CDC as well as by others which contain more information regarding HIV infection as the cause of AIDS.


  • The Controversy over HIV and AIDS
    The contention of a small number of so-called "dissenters" that HIV is not the cause of AIDS has recently received international attention, thanks to South African President Thabo Mbeki's public questioning of the scientific evidence linking the virus to the disease. In 1994, Science correspondent Jon Cohen conducted a thorough examination of the arguments of the leading dissidenter, virologist Peter Duesberg. Science is making those articles available free of charge in view of the serious public health implications of this debate.

  • Blattner W, Gallo RC et al. HIV causes AIDS. Science 1988;241:515-16.

  • Duesberg P. HIV is not the cause of AIDS. Science 1988;241:514-17.

  • Weiss R. How does HIV cause AIDS? Science May 1993;260:1273-79.

  • Winkelstein W, Wiley J, Padian N, et al. The San Francisco men's health study: continued decline in HIV seroconversion rates among homosexual/bisexual men. Am J Public Health 1988;78:1472-74.

How long does it take for HIV to cause AIDS? (Download PDF Version) Return to top

Since 1992, scientists have estimated that about half the people with HIV develop AIDS within 10 years after becoming infected. This time varies greatly from person to person and can depend on many factors, including a person's health status and their health-related behaviors.

Today there are medical treatments that can slow down the rate at which HIV weakens the immune system. There are other treatments that can prevent or cure some of the illnesses associated with AIDS, though the treatments do not cure AIDS itself. As with other diseases, early detection offers more options for treatment and preventative health care.

What is meant by "Asymptomatic Period"? Return to top

It is possible to be infected with HIV without showing symptoms of illness. An individual can transmit the virus to others, even if he or she shows no symptoms. In the United States, an estimated 650,000 to 900,000 persons currently are infected with HIV.

Many people do not develop any symptoms when they first become infected with HIV. Some people, however, have a flu-like illness within a month or two after exposure to the virus. They may have fever, headache, malaise, and enlarged lymph nodes (organs of the immune system) easily felt in the neck and groin. These symptoms usually disappear within a week to a month and are often mistaken for those of another viral infection.

More persistent or severe symptoms may not surface for a decade or more after HIV first enters the body in adults, and within two years in children born with HIV infection. This period of "asymptomatic" infection is variable, however. Some people may begin to have symptoms in as soon as a few months, whereas others may be symptom-free for more than 10 years. During the asymptomatic period, however, HIV is actively infecting and killing cells of the immune system. HIV's effect is seen most obviously in the blood levels of CD4+ T cells (also called T4 cells) -- the immune system's key infection fighters. The virus initially disables or destroys these cells without causing symptoms.

What is meant by "Window Period"? Return to top

Many people express concern regarding the length of time following infection for an individual to develop detectable antibodies to HIV. This is known as the "Window Period". According to the CDC, most persons infected with HIV develop antibodies against the virus within three months after the infection. This time period for antibody development is based on the results of numerous studies of persons with hemophilia, persons who received transfusions from HIV-infected individuals, and spouses of both these groups; homosexual men; and occupationally exposed health-care workers. The CDC studies indicate that it is highly unlikely that HIV infection would go unrecognized for prolonged periods (over six months) in persons who are infected.

For additional information, you may wish to refer to the following journal articles:

  • Peterson LR et al. Duration of time from the onset of HIV type 1 infectiousness to development of detectable antibody. Transfusion 1994;34(4):283-9.

  • Alcabes P, Munoz A, Friedland GH. Incubation period of HIV. Epidemiologic Reviews 1993; 15(2): 303-18.

  • Coutlee F., Olivier C., Cassol S., Voyer H., Kessous-Elbaz A., Saint-Antoine P., He Y., Fauvel M. Absence of prolonged immunosilent infection with human immunodeficiency virus in individuals with high-risk behaviors. American Journal of Medicine. 96(1):42-8, 1994 Jan. 94136505

What is the natural history of HIV disease? Return to top

The period between infection with HIV and development of AIDS is usually variable and long. During the "incubation" or asymptomatic period, a person is infected with the virus but is not ill. Signs and symptoms that HIV-infected persons may show first include generalized lymphadenopathy, tiredness, weight loss, loss of appetite, fever, diarrhea, night sweats, and yeast infections. The term "symptomatic HIV infection" should be used in place of "AIDS Related Complex (ARC)," a term which is now obsolete. For both clinical and research purposes, a patient's condition can be more accurately described by a listing of symptoms and laboratory evidence of infection.

Why a small minority of HIV-infected people have remained healthy for many years without loss of immune function remains unknown. However, researchers at the National Institute of Allergy and Infectious Diseases (NIAID) and their colleagues reported in the January 26, 1995 issue of New England Journal of Medicine that 15 HIV-infected volunteers with nonprogressive HIV disease had very low levels of HIV in their blood and lymph nodes, even though viral replication was persistent. The internal structure of these individuals' lymph nodes, unlike those of most people with HIV infection, appeared essentially undamaged, and their immune function remained virtually unimpaired.

The investigators defined long-term non-progressors as individuals who had been HIV-infected for seven or more years, had stable CD4+ T cell counts of 600 or more cells per microliter of blood, no HIV-related diseases, and no previous antiretroviral therapy. CD4+ T cells are the critical immune system cells targeted by HIV and typically depleted during the course of HIV infection. In healthy adults, the normal CD4 T-cell count is greater than 1000 cells per microliter of blood.

What is the primary impact of HIV on the immune system? Return to top

In a healthy person, the normal range of certain white blood cells called T4 lymphocyte cells (helper cells) is usually between 600 and 1200 (T4) cells per cubic millimeter (cells/mm3). (The range depends on the test used.) When human immunodeficiency virus (HIV) enters the bloodstream, it primarily infects T4 cells. Asymptomatic individuals infected with HIV usually have a lower than normal T4 cell count, and people with AIDS generally have between 0 and 500 T4 cells/mm3. The number of T8 lymphocyte cells (suppressor cells) in an HIV-infected person usually stays about the same. Because the number of T4 cells is low, the total T-cell count is lower than normal, and the T4/T8 cell ratio is lower than the usual 2 to 1 ratio.

HIV infection not only reduces the number of T4 cells, it can also impair a T4 cell's functioning. HIV-infected individuals with very low T4 cell counts tend to have more serious infections. Therefore, regular immunologic tests to determine T4, T8, and total T-cell counts can be an important element in monitoring the health of an HIV-infected person.

How did HIV get its name? Return to top

The AIDS virus has been variously termed human T-lymphotropic virus type III (HTLV-III), lymphadenopathy-associated virus (LAV), AIDS-associated retrovirus (ARV), and human immunodeficiency virus (HIV). The designation "human immunodeficiency virus" (HIV) has been accepted by a subcommittee of the International Committee for the Taxonomy of Viruses as the appropriate name for the retrovirus that causes AIDS (Science l986;232:697).

What is HIV? Return to top

HIV (human immunodeficiency virus) is the virus that causes AIDS. This virus is passed from one person to another through blood-to-blood and sexual contact. In addition, infected pregnant women can pass HIV to their baby during pregnancy or delivery, as well as through breast-feeding. People with HIV have what is called HIV infection. Most of these people will develop AIDS as a result of their HIV infection.

These body fluids have been proven to spread HIV:

  • blood
  • semen
  • vaginal fluid
  • breast milk
  • other body fluids containing blood
These are additional body fluids that may transmit the virus that health care workers may come into contact with:
  • cerebrospinal fluid surrounding the brain and the spinal cord
  • synovial fluid surrounding bone joints
  • amniotic fluid surrounding a fetus

Where did HIV come from? Return to top

Findings presented at the 6th Conference on Retroviruses and Opportunistic Infections held in Chicago from January 31 to February 4, 1999, provide the strongest evidence to date that HIV-1 originated in non-human primates, probably chimpanzees. Researchers from the University of Alabama at Birmingham presented evidence identifying a new isolate of a retrovirus affecting a chimpanzee subspecies (Pan troglodytes troglodytes) and showed that this and other chimpanzees isolates are related to the different groups of HIV-1 affecting humans. According to Dr. Hahn and colleagues, the establishment of HIV-1 in humans is likely to have resulted from cross-species transmission.

This new knowledge can lead to a better understanding of the evolution of HIV-1, provide insight into species-to-species transmission of viruses, and increase our understanding of infectious disease emergence. Deeper understanding of strain evolution could in the longer term be of relevance to the development of diagnostic assays and vaccines.

Two types of HIV have been identified to date: HIV-1 and HIV-2. HIV-1 is the predominant HIV type in the United States and throughout the world. HIV-2 is primarily found in West Africa. The origin of HIV-2 has been identified as being another monkey species, the sooty mangabey (Cercocebus atys); Dr. Hahn also played a key role in that research.

Statement taken from Kevin De Cock, Director, Division of HIV/AIDS Prevention, Surveillance and Epidemiology National Center for HIV, STD, and TB Prevention. Released in the CDC Update: CDC Statement in Response to Presentation on Origin of HIV-1 at 6th Conference on Retroviruses and Opportunistic Infections.

For additional information, you may want to refer to the following journal articles:

  • BH Hahn, et al. Origin of HIV-1 in the Chimpanzee Pan Rroglodytes Troglodytes. Nature 1999; 397: 436-41.

  • RA Weis and RW Wrangham. From Pan to Pandemic. Nature 1999; 397: 385-6.

How can I tell if I'm infected with HIV? What are the symptoms? (Download PDF Version) Return to top

The only way to determine for sure whether you are infected is to be tested for HIV infection. You cannot rely on symptoms to know whether or not you are infected with HIV. Many people who are infected with HIV do not have any symptoms at all for many years.

The following may be warning signs of infection with HIV:

  • rapid weight loss
  • dry cough
  • recurring fever or profuse night sweats
  • profound and unexplained fatigue
  • swollen lymph glands in the armpits, groin, or neck
  • diarrhea that lasts for more than a week
  • white spots or unusual blemishes on the tongue, in the mouth, or in the throat
  • pneumonia
  • red, brown, pink, or purplish blotches on or under the skin or inside the mouth, nose, or eyelids
  • memory loss, depression, and other neurological disorders
However, no one should assume they are infected if they have any of these symptoms. Each of these symptoms can be related to other illnesses. Again, the only way to determine whether you are infected is to be tested for HIV infection.

Similarly, you cannot rely on symptoms to establish that a person has AIDS. The symptoms of AIDS are similar to the symptoms of many other illnesses. AIDS is a medical diagnosis made by a doctor based on specific criteria established by the CDC.

Can HIV survive in ground water, sewage, body fluids, and corpses of HIV-infected persons? Return to top

The CDC has received many inquiries concerning the survival of HIV in ground water, sewage, body fluids, and corpses of HIV-infected persons. Most of the questions posed assume that HIV can be transmitted in the environment; however, this type of transmission has not been documented. The overwhelming scientific evidence is that HIV is fragile and highly susceptible to physical and chemical agents and therefore does not survive well outside the human body.

An extensive study on the survival of HIV after drying was reported by Resnik and coworkers (Stability and inactivation of HTLV-III/LAV under clinical and laboratory environments. Journal of the American Medical Association 1986;255:1887-91). The purpose of the study was to determine the inactivation rate of HIV under experimental conditions--an objective that required the use of extremely high levels of HIV. The concentrations studied were at least 100,000 times greater than those typically found in the blood of HIV-infected persons. It is not surprising that when such high concentrations of HIV were used, the virus could be detected 1 to 3 days after drying. Upon close examination of these data and from other results that have been obtained by CDC, however, it is clear that drying causes a rapid (within 1 or 2 hours) reduction in virus concentration and renders 90 to 99 percent of the virus inactive.

Other studies have shown that HIV is rapidly inactivated by a range of physical and chemical agents such as low levels of heat, pH extremes, and a variety of chemicals. These observations, coupled with the enormous dilution factors in sewage systems, suggest that on-the-job HIV risk factors for sewage workers are virtually nonexistent. HIV is transmitted by sexual contact with an infected person, perinatally from an infected woman to her fetus or infant, through needle-sharing among intravenous drug users, and rarely, from accidents involving needlestick injuries and other blood exposures of health-care providers. Because there is no epidemiologic or laboratory evidence that HIV can be transmitted by the fecal-oral route or by air, fears associated with HIV transmission by other types of contact with sewage are not warranted.

Although there have been no specific studies of HIV survival in corpses before or after embalming, no instances of HIV transmission have been reported from an exposure incurred in performing mortuary services. The chemical germicides in embalming fluids have been tested and found to completely inactivate HIV. CDC has also published occupational infection control guidelines that apply to mortuary workers, including embalmers.

How does temperature effect the survival of HIV? Return to top

HIV is very fragile and does not survive well outside the human body. HIV is inactivated by heat and dies after 30 minutes at 56EC (132.8EF). It is also highly susceptible to physical and chemical agents.

If properly stored, HIV is very stable at low temperatures. It can last 7-10 days at 4EC (39.2EF) and months to years at -70EC (-94EF). The Centers for Disease Control and Prevention (CDC) stores purified virus preparations in liquid nitrogen (-200EC or -328EF). In all cases, the stability of HIV depends on the presence of the proper concentration of protein to protect the virus from changes in temperature.

Is there a difference in survival between cell-free virus and cell-associated virus? Return to top

In the human body, cell free HIV enters the CD4+ lymphocyte, becomes cell-associated, and reproduces, resulting in new viruses being released (cell free) where they may infect other CD4+ cells. Both cell free HIV and cell-associated virus are present in circulating blood.

Laboratory-based inactivation studies have been performed using high concentrations of virus so that it can be detected more easily (the amount of virus normally found in infected blood is difficult to detect). The concentrations of virus used are up to 100,000 times greater than the amount of virus normally present in an HIV-infected individual's blood.

In the study cited in the 1987 universal precautions document the investigators use both cell free and cell-associated concentrated virus. Overall, when the virus was subjected to drying conditions, investigators found there was a rapid decrease in the amount of virus. When they started with a high concentration of virus, there was a 90-99% reduction in the amount of virus detected after several hours. When they used pure (cell-free) concentrated HIV it could be detected up to 15 days at room temperature and 11 days at 98.6 degrees F. However, when concentrated cell-associated virus (cell-associated virus is probably more analogous to how you would encounter the virus in blood) was used it could be found for only one day. These survival times are probably related to the amount of virus used in the study (i.e., the more that you start with- the longer you are able to detect it). It is difficult to know exactly how these times would change if normal concentrations of HIV were used, but most experts would assume they would be much less.