Feline leukemia virus is a retrovirus that suppresses the immune system and causes cancer in cats, and is almost entirely preventable through vaccination and indoor housing, yet still kills hundreds of thousands of cats annually.
FeLV causes cats to suffer from immune suppression that leaves them vulnerable to secondary infections they would otherwise fight off, from progressive anemia, and from various cancers including lymphoma. Affected cats experience repeated illness, the distress of chronic disease, and eventual death from cancer or immune failure. Many affected cats are diagnosed young - sometimes as kittens - and face a shortened life of recurrent illness. The tragedy of FeLV is its almost complete preventability: vaccination of kittens, testing before introducing new cats, and keeping cats indoors effectively eliminates the risk. Every FeLV death in a vaccinated cat population represents a preventable welfare failure.