Well, AIDS is communicable and could depopulate the planet without study, and measures taken. Alzheimer's is not nearly so dangerous.

You're right that AIDS is comparitively over-funded. Breast cancer kills a lot more Americans every year. Funding comes from all over the place, however. There is no regulating world body to assign proper percentages of funding to diseases.

I think one reason that there is so much activism about AIDS is that there was a tendency on the "moral" right to label it God's wrath against the gays, IV drug users, and dark people in the Caribbean and Africa (soon to add the godless Chinese to the list). Resistance breeds resistance.