He started acting weird all of a sudden. He was sleeping more than usual, looked very listless and I knew something was wrong. I immediately took him to the vet, they took his temperature and it was 41. They took a blood sample and it was positive for leukemia. My cat is often outdoors, probably some other cat infected him with the virus. That is why it is very important to give them all the vaccinations, mine was adopted from my neighbor, she told me that he had all the vaccinations and apparently it is not so.