Is there a leash law where you are? If not, and the dog was on a public right-of-way you could maybe have a problem if you shot the dog. As noted by several others, REPORT aggressive off-leash dogs. Establish a track record.

That being said you have a legitimate medical issue where dog bites would be very bad for you. Does the dog have it's shots? Does it wear a collar with tags so you KNOW or at least reasonably suspect it's had its shots? Being able to articulate all this if push comes to shove is important.