If Harley has FLV, odds are the momma cat does, too. If she hasn't been tested, she should be ASAP. If she's negative, go ahead and get her vaccinated. If she's positive, there is no cure and she'll infect other cats she comes in contact with.
Been there, done that and it was no fun. I lost 13 of 15 cats to the disease.
FLV can't be transmitted to humans or to other animals. Neither can Feline AIDS. There is a canine version of that, and an equine version though.
Oh God, this is unphantomable! Don't have Harley's Mama here but felt it could be genetic and her immune system is genetically and inherently weak. Now I don't know why her 3 counts are off the charts. But if I did I'd be keeping her isolated. The vet said if my other three have been exposed, its too late already? Harley is isolated in one room now. And I'm taking her to another vet for follow-up.