The scientific name for the disease is Dermatophytosis. It is not a worm, rather a fungus. There are three types of common fungi that cause the condition. Microsporum Canis (almost all cases in cats are from this fungi), Microsporum Gypseum and Trichophyton Mentagophyte (which often occurs due to exposure to rodents and their burrows–particularly important to feral cats).
The skin lesions that appear with Ringworm are variable, and do not necessarily form a ring. There will be hair loss, usually in small patches at first. As time goes on the patches may disappear or appear at other locations on the skin. There might be scratching due to itchiness. If the hair loss occurs on the face or feet there is a chance it is due to digging habits or exposure to rodents. You cannot diagnose simply from the loss of hair because it mange and allergic reactions can look exactly the same. It is difficult to see such hair loss in a feral unless you are closer to it, feeding times are an excellent time to take a good look at your colonies hair condition. You may also see: broken and brittle hair, partial or patchy hair loss, scales or crusts, multiple scales and areas of self-mutilation due to itching. Rarely, infection will go into deeper tissues resulting in inflamed lesions.
Is nearly impossible as fungi are everywhere and highly resistant to virtually everything. Clorox diluted 1:10 kills approximately 20% of the spores. Outside, where most ferals live, there is nothing to be done to prevent infection.
There are a number of drugs, topical and systemic, which are useful for treating clinical ringworm infection in individual cats. Aside from topical and systemic antifungal class of drugs, such as itraconazole, there is more recent information which suggests that lufenuron (Program) is inconsistently efficacious, but it is very safe! The vaccine can alleviate the clinical signs BUT does not eliminate infection!