You're right on the money (about the money). It's not only density that will determine which "cured" diseases will rise up again to cause problems. Typically, diseases that strike developed countries get most of the research funding. Flu is huge because it threatens the developed world, and the funding is pouring out from a whole alphabet of government agencies.

TB and malaria are both making drug-resistant comebacks around the world but the research is lagging due to funding, especially at the point where you need clinical trials. The NIH has made both of these priorities (as has the Gates Foundation), but they only pay for the initial lab work. Drug companies typically pay for the more expensive R&D phase, which includes the trials, but without a clear model of how they will make a profit they won't invest the funds. There is no profit in curing poor people in India and Africa.

Until kids in Florida start getting multi-drug-resistant malaria there won't be any rush, but by then we'll be 10 years behind the curve.