If this is a new pet and you are unsure of the history, if your cat is outdoor only or indoor/outdoor or is exposed to another cat that goes outside it is a good idea to test for FeLV and FIV.  Both infectious diseases are easily spread from cat to cat.  It can be transmitted through the litterbox, grooming each other, food and water bowls, through feces, and lactating mothers pass in onto nursing kittens.  FIV is primarily transmitted through bite wounds.