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How do we know that HIV causes AIDS?
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It's a matter of science

In 1981, the first cases of severe immune system deterioration were recognized when several young men developed unusual infections and cancer. The new disease was later named "AIDS." At that time, no one knew what was causing the disease. Since then, science has shown that the human immunodeficiency virus (HIV) is the cause of AIDS. We know this because people who become infected with the virus can develop AIDS, and medicines that act against the virus can prevent infected people from getting sick. As HIV infection progresses, it weakens a person's ability to fight off diseases. By attacking the immune system, the virus leaves people more susceptible to other diseases. When a person with HIV develops one of several additional diseases, or when a person's immune system shows marked deterioration, as measured by a very low CD4 cell count, that person is considered to have AIDS.

Some individuals and groups say that AIDS is not caused by HIV. These claims can be really harmful. If people are led to believe that they do not need to reduce their risk of getting or spreading HIV, they could become infected or pass the virus to others. Also, if people with HIV think they do not need to get medical care or consider treatment, they could become very sick and develop AIDS.

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