==Of course such people, even without being HIV-positive, are going to fall prey to many of the same diseases as people with AIDS...(so) It’s not surprising that people with weakened immune systems due to other causes will find themselves in a similar situation.
So how do we tell which is which??? In other words, let’s suppose a HIV-positive drug addict comes in with a weakened immune system and suffering from an AIDS defining disease. How could we possibly indentify the underlying cause of his disease? It could be HIV, it could be drugs, it could be malnutrition. How would we tell the difference?
In the case of the HIV-infected homeless drug addict, any doctor can tell you that all of the various things involved have served to compromise his immune system. The HIV/AIDS have just made an already bad situation even worse.
As a counterexample, look at someone like Magic Johnson who is HIV+, but lives a lifestyle of diet, medication and exercise that allows him to keep the disease in check more or less indefinitely. He's the flipside of the drug-addicted homeless person who is HIV+
To figure out the effect of HIV infection, you compare one group of drug addicts who are not HIV positive with a similar group of drug addicts who are, then look at the relative rate of diseases arising from compromised immune systems. You'll find instances of such diseases in both groups, but the rate for the HIV infected drug addicts will be, over the long-term, significantly higher.
You are quite correct that living a life of drug abuse and promiscuous homosexual orgies (with all of the attendant STDs and other health issues) is bad for one's immune system. But that lifestyle, even if it leads to getting certain AIDS-like diseases, is not the cause of AIDS. It can simply lead to health situations that are similar to AIDS, however.