www.niaid.nih.gov/factsheets/evidhiv.htm
The Evidence That HIV Causes AIDS, NIAID Fact Sheet Created November 1994. The Evidence That HIV Causes AIDS BACKGROUND The acquired immunodeficiency syndrome AIDS was first recognized in 1981 and has since become a major worldwide pandemic. By leading to the destruction and/or functional impairment of cells of the immune system, notably CD4 T cells, HIV progressively destroys the bodys ability to fight infections and certain cancers. An HIV-infected person is diagnosed with AIDS when his or her immune system is seriously compromised and manifestations of HIV infection are severe. Most other AIDS-defining conditions are also opportunistic infections which rarely cause harm in healthy individuals. A diagnosis of AIDS also is given to HIV-infected individuals when their CD4 T-cell count falls below 200 cells/cubic millimeter mm 3 of blood. Healthy adults usually have CD4 T-cell counts of 600-1,500/mm 3 of blood. In HIV-infected children younger than 13 years, the CDC definition of AIDS is similar to that in adolescents and adults, except for the addition of certain infections commonly seen in pediatric patients with HIV.
As of the end of 2000, an estimated 361 million people worldwide 347 million adults and 14 million children younger than 15 years were living with HIV/AIDS. Through 2000, cumulative HIV/AIDS-associated deaths worldwide numbered approximately 218 million 175 million adults and 43 million children younger than 15 years. In the United States, an estimated 800,000 to 900,000 people are living with HIV infection. As of December 31, 1999, 733,374 cases of AIDS and 430,441 AIDS-related deaths had been reported to the CDC. AIDS is the fifth leading cause of death among all adults aged 25 to 44 in the United States. Among African-Americans in the 25 to 44 age group, AIDS is the leading cause of death for men and the second leading cause of death for women UNAIDS .
This document summarizes the abundant evidence that HIV causes AIDS. Questions and answers at the end of this document address the specific claims of those who assert that HIV is not the cause of AIDS. EVIDENCE THAT HIV CAUSES AIDS HIV fulfills Kochs postulates as the cause of AIDS. Among many criteria used over the years to prove the link between putative pathogenic disease-causing agents and disease, perhaps the most-cited are Kochs postulates, developed in the late 19th century. Kochs postulates have been variously interpreted by many scientists, and modifications have been suggested to accommodate new technologies, particularly with regard to viruses Harden .
However, the basic tenets remain the same, and for more than a century Kochs postulates, as listed below, have served as the litmus test for determining the cause of any epidemic disease: Epidemiological association: the suspected cause must be strongly associated with the disease. Isolation: the suspected pathogen can be isolated - and propagated - outside the host. Transmission pathogenesis: transfer of the suspected pathogen to an uninfected host, man or animal, produces the disease in that host. With regard to postulate 1, numerous studies from around the world show that virtually all AIDS patients are HIV-seropositive;
With regard to postulate 2, modern culture techniques have allowed the isolation of HIV in virtually all AIDS patients, as well as in almost all HIV-seropositive individuals with both early- and late-stage disease. In addition, the polymerase chain PCR and other sophisticated molecular techniques have enabled researchers to document the presence of HIV genes in virtually all patients with AIDS, as well as in individuals in earlier stages of HIV disease. Postulate 3 has been fulfilled in tragic incidents involving three laboratory workers with no other risk factors who have developed AIDS or severe immunosuppression after accidental exposure to concentrated, cloned HIV in the laboratory. In all three cases, HIV was isolated from the infected individual, sequenced and shown to be the infecting strain of virus. In another tragic incident, transmission of HIV from a Florida dentist to six patients has been documented by genetic analyses of virus isolated from both the dentist and the patients. The dentist and three of the patients developed AIDS and died, and at least one of the other patients has developed AIDS. Five of the patients had no HIV risk factors other than multiple visits to the dentist for invasive procedures OBrien, Goedert .
Kochs postulates also have been fulfilled in animal models of human AIDS. Chimpanzees experimentally infected with HIV have developed severe immunosuppression and AIDS. In severe combined immunodeficiency SCID mice given a human immune system, HIV produces similar patterns of cell killing and pathogenesis as seen in people. HIV-2, a less virulent variant of HIV which causes AIDS in people, also causes an AIDS-like syndrome in baboons. More than a dozen strains of simian immunodeficiency virus SIV, a close cousin of HIV, cause AIDS in Asian macaques. In addition, chimeric viruses known as SHIVs, which contain an SIV backbone with various HIV genes in place of the corresponding SIV genes, cause AIDS in macaques. Further strengthening the association of these viruses with AIDS, researchers have shown that SIV/SHIVs isolated from animals with AIDS cause AIDS when transmitted to uninfected animals ONeil et al.
AIDS and HIV infection are invariably linked in time, place and population group. Historically, the occurence of AIDS in human populations around the world has closely followed the appearance of HIV. In the United States, the first cases of AIDS were reported in 1981 among homosexual men in New York and California, and retrospective examination of frozen blood samples from a United States cohort of gay men showed the presence of HIV antibodies as early as 1978, but not before then. Subsequently, in every region, country and city where AIDS has appeared, evidence of HIV infection has preceded AIDS by just a few years CDC .
Many studies agree that only a single factor, HIV, predicts whether a person will develop AIDS. Other viral infections, bacterial infections, sexual behavior patterns and drug abuse patterns do not predict who develops AIDS. Individuals from diverse backgrounds, including heterosexual men and women, homosexual men and women, hemophiliacs, sexual partners of hemophiliacs and transfusion recipients, injection-drug users and infants have all developed AIDS, with the only common denominator being their infection with HIV NIAID , 1995. In cohort studies, severe immunosuppression and AIDS-defining illnesses occur almost exclusively in individuals who are HIV-infected. For example, analysis of data from more than 8,000 participants in the Multicenter AIDS Cohort Study MACS and the Womens Interagency HIV Study WIHS demonstrated that participants who were HIV-seropositive were 1,100 times more likely to develop an AIDS-associated illness than those who were HIV-seronegative. These overwhelming odds provide a clarity of association that is unusual in medical research MACS and WIHS Principal Investigators , 2000. In a Canadian cohort, investigators followed 715 homosexual men for a median of 86 years. Every case of AIDS in this cohort occurred in individuals who were HIV-seropositive. No AIDS-defining illnesses occurred in men who remained negative for HIV antibodies, despite the fact that these individuals had appreciable patterns of illicit drug use and receptive anal intercourse Schechter et al.
Prior to the appearance of HIV, AIDS-related conditions such as Pneumocystis carinii pneumonia PCP, Kaposis sarcoma KS and disseminated infection with the Mycobacterium avium complex MAC were extraordinarily rare in the United States. In a 1967 survey, only 107 cases of PCP in the United States had been described in the medical literature, virtually all among individuals with underlying immunosuppressive conditions. Before the AIDS epidemic, the annual incidence of Kaposis sarcoma in the United States was only 02 to 06 cases per million population, and only 32 individuals with disseminated MAC disease had been described in the medical literature Safai .
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