More scientific evidence has surfaced to show that while mittens may be your sweet angel, letting her roam outside is also a big threat to biodiversity.
While I definitely lean towards not letting cats roam free (not even possible in our current living situation), I'd say that FeLV and FIP scare me way more than FIV. FIV+ cats can live relatively normal, long lives when living in safe, caring homes with some very basic precautions
FIV is primarily transmitted via saliva in deep bite wounds when one cat bites another, and the majority of those bites occur when an intact male cat bites another intact male cat, so if your cats are spayed/neutered, the risk of contracting FIV drops significantly. With proper slow introductions between altered cats, FIV+ and FIV- cats can live together with minimal risk of transmission.
I only mention this because there's so many FIV+ cats that get stuck in the shelters due to stigma, and wish I could save them all.