Bringing it down to scale like this is a familiar concept. We think that stealing is bad and people should be punished, but when someone steals our car, we have a much stronger reaction. The same holds true of something like AIDS or Malaria or hunger. We are in agreement that those things are bad, but when your best friend gets HIV, or your mom gets malaria, or it is your daughter who doesn't get to eat tonight, aren't you going to spring into action? I know I am.
To look more accurately at what HIV/AIDS does, I think that we should differentiate between what happens when you contract HIV/AIDS here in the US and what happens if you were to contract it as a citizen of (for example) sub-Saharan Africa. (Please understand that these are generalizations, but that does not mean they don't happen. See World Vision for more information on the situation in other countries.)
Do you see the discrepancy here? HIV will infect anyone -- location is irrelevant. It is after contracting the virus that something goes wrong. Why should a person here live a longer life than someone who is in Malawi or Uganda or Haiti if they have the exact same disease?
I can't think of a good reason either. It's because there is no good reason. It comes down to differences in the environment of these two people.
The American lives in a place where education on sexually transmitted diseases is important and frank. The stigma against those who contract HIV/AIDS is minimal if not nearly non-existent. Treatment is available and the likelihood of infection and illness is very slim thanks to our standards of health for food and drinking water. But the African may not be aware that they can contract HIV/AIDS through unprotected sex, and if they contract the disease, might be too scared to see a doctor. Medication for AIDS is scarce, and the rate of infection and illness high because of dirty water and various pests and parasites.
Now what did you notice about the situation of the African?
Every one of those problems is one that can be resolved.