Feline immunodeficiency virus is a lentivirus causing immune suppression in cats, but with proper management FIV-positive cats can live comfortable, long lives - though the diagnosis still leads to unnecessary euthanasia in many shelters.
FIV-positive cats suffer primarily from the secondary consequences of immune suppression - recurrent infections, dental disease, and eventually cancer. The virus itself causes little direct harm for potentially years to decades. The welfare tragedy of FIV lies largely in how these cats are treated by humans: approximately half of animal shelters euthanize FIV-positive cats despite guidelines from veterinary organizations stating this is not justified. These cats can live comfortable, happy lives with appropriate management and monitoring. The profound mismatch between the actual welfare implications of FIV and the shelter response to diagnosis represents a massive, unnecessary welfare harm caused by outdated practices rather than the disease itself.