Reasons why people say that AIDS/HIV doesn't exist
I think no one can really deny the existence of HIV or AIDS, just a search on google scholar will show >1,500,000 hits for each of those terms, and ask (hopefully any) doctor and they will say it does, though AIDS denialists do debate whether HIV causes AIDS. This paper explains the process of HIV causing AIDS. Further, AIDS denialists have not offered up well supported alternatives as to how AIDS arises without HIV being involved.

I think however that there is more debate fuelled by conspiracy theorists as to how the disease came about. This article from TIME magazine on conspiracy theories is a short piece about it, there are two main conspiracy theories:

1) The west invented the disease to "decimate the African population."

2) The US government invented the disease in the 70's and injected gay men to stop the spread of homosexuality.

You should read this section on the critical response to the film House of Numbers, it seems to be thought that the film was driven by funding wishing to promote conspiracy theory and interviews were edited in such a way as to help portray that message.

Caution, the following is my own personal opinion ;)


  To most these seem like pretty outlandish claims and most people do ignore them - apart from anything, placing blame on someone won't stop the continuing spread of this horrendously devastating disease so the conspiracies are quite pointless. I personally think these conspiracies are driven by the uneducated, the scared, and those who want to "bring down the man &amp; corporate/capitalist western society led by a powerful minority" etc..


Back to the science: The people who discovered the disease (Dr. Robert Gallo of the National Cancer Institute and Dr. Luc Montagnier of the Pasteur Institute in Paris) along with the majority of the scientific community say:


  "the virus jumped from monkeys to humans some time during the 1930s" 


Molecular analysis of the genetic sequence for the virus supports the theory that the jump from apes to humans was around 1920-1930. Some other studies support the jump being made around this time though some estimate it to be a little earlier, this is due to estimated rates of mutation being used to calibrate molecular clocks.

I recommend taking a look at AIDStruth.org and more specifically start with this page (thanks for the link @MattDMo) if you have watched House of Numbers.