One major characteristic of a good biological weapon is that it kills the other guys, not your own people. It can be quick and disappear when the people who have it die or just disspipate. It has to be containable, though, either because it is short-term and confined to a specific area or because your guys have a vaccine or something that will cure it. Somebody who intends to wipe out a poplulation won't use a biological agent that goes undetected in a handful of people for years and can travel around the world and spread, infecting more small numbers of people, until the numbers aren't so small anymore. Not unless they, and their children and grandchildren, can be protected.

I think the modern belief in the cause of all disease being an 'unhealthy lifestyle' is the equivalent of the older belief in 'bad spirits'. Not taking care of your body in DOES cause problems, but not all of them. These beliefs allow us to feel somewhat immune, though. All we have to do is maintain a healthy lifestyle, and we won't get sick and die like they do.

When you're trying to figure out if a weakened immune system is a result or a cause of AIDS, do consider people that were healthy until they contracted the virus.
Consider that the virus reproduces in T-cells which are the body's first line of defense, the 'general purpose" immune cells. It destroys those cells in the process.
Consider that a low level of T-cells is one of the factors needed for a diagnosis of AIDS.
Now, explain why those cells started out at a normal level and then dropped to the point where the individual was susceptible to opportunistic infections.

Most people are used to hearing about those who get sick and then are diagnosed with AIDS. They're used to hearing about injectable drug users, the sexually promiscuous, and people who otherwise probably aren't tested when they're healthy. What I've seen are the healthy ones who've been found to be HIV positive on tests, and subsequently, I've seen their bodies slowly and horribly give up on them, despite all they tried to do to prevent it. You're saying they could have done more, that they should have done more. You're saying it was their fault they got AIDS.