(For context, see part 1 below.) I’m skeptical of government funding of any medical research, but I can think of at least two reasons (besides Eugene’s) why it makes sense to give greater funding to HIV/AIDS than breast cancer or heart disease, if funding is measured on a per-victim basis.
First, HIV/AIDS tends to strike people down in the prime of life, whereas breast cancer and heart disease strike down older people. That means that lives saved by means of HIV/AIDS correspond to a greater number of life-years saved. If the government were interested in saving the most number of life-years instead of the most number of lives, it would be justified in spending a greater amount per HIV/AIDS-victim than per victim of diseases that strike later in life. (I don’t have specific figures, so I can’t say whether the difference in average age at death is large enough to justify the actual disparity in funding. If anyone can find specific figures, please email them to me.)
Second, any treatment that prevents, cures, or inhibits the transmission of HIV/AIDS create positive externalities; this is a result of the communicable nature of HIV/AIDS. For example, if someone is vaccinated against a disease, other people who have not been vaccinated also benefit, because their chance of being exposed to the disease is lower when fewer members of the population have it. The same cannot be said of non-communicable conditions like heart disease or cancer; the fact that you’ve been treated for cancer doesn’t make me any less likely to get cancer. Since there is a stronger case for subsidizing activities with positive externalities, it makes sense that HIV/AIDS would get greater funding than non-communicable conditions. (The fact that a conscientious person can generally avoid exposure to HIV/AIDS, by practicing abstinence or monogamy, using condoms, etc., weakens but doesn’t eliminate the argument. First, even for those who practice safe sex, some residual chance of exposure still remains, especially in the case of healthcare workers. Second, all these practices are burdens for at least some members of the population, and lower likelihood of exposure would reduce the need to use them.)
I don’t mean to imply that either of these arguments is a definitive argument in favor of funding research on HIV/AIDS or any other form of medical research. There are good public choice reasons to resist any involvement of government in setting scientific priorities. But for the time being, government funding isn’t going anywhere, so we might as well ask whether the priorities chosen are justified. With regard to the seemingly disproportionate funding of HIV/AIDS over other illnesses, prima facie considerations indicate the answer may well be yes.