I have had several people over the years ask me if ringworm is really worms. The answer is, NO. Ringworm is caused by a fungal infection. There are really not that many diseases that can be spread from one species of animals to another or from animals to people (zoonotic). However, ringworm is one of the diseases that CAN be spread. Cats and dogs can give ringworm to each other and both of them can give it to people.
In humans, it causes a lesion on the skin that is usually circular and very itchy (pruritic). In dogs, it causes patchy hairloss (alopecia) that can occur anywhere on the body. The skin in the area of the alopecia is usually flaky. Dogs are usually not pruritic.
Cats are the tricky ones. Some cats will have patchy hairloss. Usually the hairloss is localized around the mouth, eyes, and on the ears. However, there are cats that are asymptomatic, which basically means that they have the fungus on their fur, but don't have any lesions. These cats are still contagious! So, if you suddenly come down with a ringworm lesion and you recently had contact with a cat. The cat was probably the source, even if it appeared healthy.
The fungus that causes ringworm can also survive very well in the environment including the dirt. The fungal spores can also travel through the air and hide in places like air conditioning ducts for very long periods. It is very difficult to get rid of ringworm once it has entered an environment. The fungus is very susceptible to household cleaners. So, it is pretty easy to get kill it on cleanable surfaces. The problem is the places that it can hide that we don't usually clean.
If your pet is diagnosed with ringworm, it is very important to limit the areas that it goes and to keep it isolated from other pets and also from children. Ringworm is a treatable disease, but as I said, it is very hard to get rid of once it gets into an environment.