Research on infectious disease & co. is a big part of military research because servicemen operate all across the world, including areas where they are exposed to diseases that are uncommon in the US, or that have been eradicated. That is, of course, on top of all the diseases that fall under "chemical warfare" and for which you have to be able to treat your people (both military and civilian), and afflictions associated with crowded areas in general (besieged cities and the like).

I obviously don't know what the labs mentioned by the parent poster were doing but the fact that the US DoD (or any country's department of defense) does medical research should be about as surprising as the fact that they do research on explosives.