>>4782187>Isn’t FIV way less lethal than human AIDS though?Yes, but you still don't want to infect your other kitty. Long term treatment does require extra vet care and usually drugs. On the other hand, FIV isn't quite as easy to pass along as urban legend has it. Still a risk, but can be treated and most cats will live most of their lives as if they didn't have it, albeit with slightly to somewhat compromised immune systems that can make them vulnerable to other disease & infection.
There's also other diseases, FIV is just sort of short-hand for "ferals may be sick with something that can infect other cats." Not necessarily always FIV. There are also parasite issues, internal through diet and of course external like fleas & mites & such.
Of note, scratching a cat at the base of their tail is pretty intimate. I'd usually start off with a couple light pats to test the waters and slowly get a feel for how an individual reacts to it. Strays/barn cats aren't always socialized to "go there" immediately and should be approached slowly & gently. It's a big leap of trust. Thus the attack mode. But she seems like she chilled out quickly, so it's a good start. You might consider trying to initiate play, such as simple string or something else that's interactive to hep initiate bonding & trust (beyond food & friendship scritches).
Good luck anon, to all of you. Hopefully this will turn out so everyone has a happy home together.