Feline leukemia virus is one of the most dangerous infectious diseases affecting cats today. It's spread from cat to cat through tears, saliva, urine, or droppings. Even if your cat becomes infected, there's a slight chance that its immune system will take care of the disease. There's no medical cure. An infected cat could exhibit any of a dozen or more symptoms, including depression, weight loss, diarrhea, or constipation. Feline leukemia virus also lowers the cat's resistance to other diseases, any of which could prove fatal in its weakened state. Prevention is the best course of action. Kittens should be vaccinated when they're nine or ten weeks old, with a booster a month later. The vaccines are not 100 percent effective, so even cats who've had their shots should be kept away from infected animals.