The answer is simply the first paragraph of [1]:

> The U.S. Department of Defense’s Biological Threat Reduction Program collaborates with partner countries to counter the threat of outbreaks (deliberate, accidental, or natural) of the world’s most dangerous infectious diseases. The program accomplishes its bio-threat reduction mission through development of a bio-risk management culture; international research partnerships; and partner capacity for enhanced bio-security, bio-safety, and bio-surveillance measures. The Biological Threat Reduction Program’s priorities in Ukraine are to consolidate and secure pathogens and toxins of security concern and to continue to ensure Ukraine can detect and report outbreaks caused by dangerous pathogens before they pose security or stability threats

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.

The DoD funds a lot of medical research to improve combat readiness. When I was an academic researcher, we were working on a project that was partially DoD funded to develop a diagnostic device which was meant for personalizing medication for patients undergoing cardiac procedures, but the military was interested in using it for developing personalized versions of QuikClot for soldiers who experienced battlefield trauma.

I also worked for a company which had several novel therapeutics for agents commonly used in bioweapons in their pipeline and the DoD was funding parts of that as well.

To defend against biological weapons. The DoD is responsible for defending the US against all kinds of attacks -- conventional, nuclear, chemical, and biological -- even if it doesn't use all of those weapon types itself.

The US DoD funds a lot of peaceful science at labs around the world. This is often misinterpreted by laymen as "weapons labs" when they are just normal civilian labs. Not all research funded by the military is directly applicable to weapons. A lot of times its applicable to defense against said weapons. Which also has application in the civilian medical field.

As evidenced by covid 19, pandemics are a matter of national security. I like to think that the DoD is interested in developing novel ways to fight novel pathogens, rather than use them as weapons. That said, I am under no illusion that the DoD or any government institution should be simply taken at their word either.

The other comments already answer this question. But I'd like to add just how massive the DoD is. It's not like how one might imagine a typical military, where the government is basically funding an entity to pay soldiers' salaries, pay for their equipment, and basic necessities and not much else. The DoD on a macro level doesn't really skimp on anything, from research on how climate change will affect its mission to dealing with overwhelming quantities of rust.

Plus historically, there are plenty of wars where disease was more deadly than enemy combatants.

DARPA funds all kinds of stuff. The national defense angle I think should be obvious at this point post COVID.

Because medical research is useful?

You're asking this during Covid?

cause they were reducing threat of COVID in UA Armed Forces (first link)