a weapon released without a means to target and control damage is generally less effective than one you can make sure isn't pointed at your feet.

lyme disease 'immunity' is spotty at best, and not well correlated to a specific group of people or traits; so while I could imagine its' use as a weapon, it seems that from a military strategy point of view that there should be better options.

there are plenty of diseases out there with what seems to be a racial component to immunology, I would imagine those are the ones with a lot of bio weapons interest towards them.