In the case of HIV there are likely a variety of different functions going on: First political organization: dealing with HIV became connected to the gay rights movement, especially when religious figures and politicians who were not happy with the gay rights movement said that gays deserved it or that it was punishment from God or otherwise mocked what was happening.

Second, HIV has a long time from diagnosis to when it becomes AIDS. This makes it a disease where the people with it can actively take part and lobby for more funding- since the primary treatments put the disease merely in check rather than curing it, the medical results make this tendency more strong rather than less strong.

Third, the massive increase in HIV cases in the 1980s made it seem like a disease that was a general threat to the population, and people are still riding that assumption.

Fourth, HIV is a disease that in principle (and sometimes in practice) can arise in a variety of different populations: the presence of people who received it from blood transfusions helped make it feel more like a disease threatening the general population (this connects in the obvious way to point three), and this combined with the presence of HIV+ babies to give a strong emotional aspect.