HIV causes AIDS. An obvious scientific fact to anyone with even a cursory knowledge of modern biology or medicine. In fact, HIV is one of the most intensely studied viruses we know and the causative link between HIV and AIDS is the most well-researched connection between a pathogen and syndrome that we know. If you do not accept that HIV causes AIDS, then you cannot accept any other causative link such as the one between rhinoviruses and colds or Yersinia and plague.
So how do we know that HIV causes AIDS? The answer is scientific evidence. Lots and lots of scientific evidence, ranging from a mechanistic understanding of how HIV infects the body and suppresses the immune system, the efficacy of antiretrovirals and dozens of other independent lines of evidence. Yet, there are still those pseudoscientific quacks and cranks who think that HIV is just a harmless passenger virus and has no relationship to AIDS, or worse that HIV even exists. Despite the evidence, these people prefer to spread misinformation online and harm the health of people with HIV.
In a chilling irony, many HIV/AIDS denialists have died of AIDS, including Christine Maggiore. After getting HIV, she became an HIV activist, but were later drawn into the conspiracy swamp of AIDS denialism. She also did not give her children any antiretrovirals or attempt to limit mother-to-child transmission. Her daughter Eliza Jane Scovill died in 2005 and Maggiore herself died in 2008. This did not put that much of a dent into the HIV/AIDS denialist movement. In fact, they attempted to rationalize it away as dying due to stress or other issues.
When HIV/AIDS denialism has gotten into governments, it has caused the premature deaths of over 330 000 people. This is because the South African President Thabo Mbeki blocked import of HIV medication by promoting the conspiracy theory that HIV/AIDS was somehow just a pharmaceutical plot to destroy the future of Africans.
Luckily, scientists have been fighting back against the primitive darkness of denialism by spreading information about the scientific evidence and how to protect yourself against sexually transmitted infections and removed many barriers to people getting access to effective medication. They have even written blog posts refuting the pseudoscience promoted by HIV/AIDS denialism and some have even shared their knowledge about the flawed claims put forward by science denialists in the scientific literature.
One such paper is entitled HIV Denial in the Internet Era and was published by Tara Smith and Steven Novella in PLoS Medicine back in 2007 (it is available in PDF format here). The review paper surveys recent developments in HIV/AIDS denialist movements, such as prominent individuals and organizations, their selective approach to scientific evidence and medical experts, the mischaracterization of science as dogmatic religion, how they believe that the mainstream scientific model for HIV and AIDS will collapse at any moment, their flawed logic of moving the goalposts and requiring more and more scientific evidence, and how they are not even internally consistent among themselves.
Smith and Novella finish by noting that medical doctors have a responsibility to ensure that the public perception of HIV and AIDS is accurate and not based on dangerous misinformation and that HIV/AIDS denialism has to be fought in the context of the larger anti-intellectual and pseudoscientific culture. If you only have time to read one paper about HIV/AIDS denialism, this is it.
Today, HIV/AIDS denialism is a marginal political movement that has lot a lot of steam since the early years of the new millennium. It survives mainly on the Internet, and it is there the main battle field lies.