Ringworm is not a worm, but a fungal infection. It is one of the few diseases that can be transmitted from cat to human and human to cat. Fungal spores are everywhere, and this fungal infection can break out with no known contact with another infected person or cat. A vaccine to treat this infection has recently become available, so if it is a problem where you live, you should ask your vet about the vaccine. Fortunately, in most cases of both cat and human, it cures itself within six or seven weeks. It is not advisable though to leave it untreated because of the possibility that it will become chronic and that is could be passed to others. This infection usually starts as a sore-looking bald circle, but if your kitty develops any sort of bald spots, have your veterinarian take a look. As the fungus grows outward, the newly infected skin forms a reddened ring around the older, central part of the lesion, which is usually paler and scaly. There are many antifungal agents that can be used on cats. Your veterinarian should see the cat as soon as possible, make the diagnosis, and prescribe the treatment.
Ringworm to Siamese Cat Health