We sometimes get cats that are positive for FIV. These cats can still live a happy life. Here is an article with some useful information on FIV cats.
https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/feline-immunodeficiency-virus-fiv
Sometimes, we also get dogs that are heartworm positive. To treat the heartworms, we always recommend the slow kill method. This is a lot safer than the fast kill method. Here are some good articles explaining the slow kill method.
https://parasitesandvectors.biomedcentral.com/articles/10.1186/s13071-017-2190-9
https://www.shenandoahrescue.org/post/understanding-heartworm-treatment-exploring-the-slow-kill-approach-with-advantage-multi