malnutrition is a real likely culprit. i had an adopted cat who had lost teeth due to mal nutrition as well. it could be an injurynor allergy as well. give kitty a real good brushing and take a close look at the spots where the fur is missing. try getting them cleaned up real well with a damp cloth, infact try a water dampened cloth all over the cat's coat. pull out the ickys. keep going with the food and water the cat should start to look normal soon. try some of the super fatty wet food it will help to put some fat back on the cat and the oils are good for the coat. maybe some tuna packed in oil. i know that'a a bit extravagant for a stray, but they will gobbel it right up! i always keep some wormer in the house for strays as well. it's a powder that i mix into the food. i never want to chance it with my own pets to let them get too close to another animal unless i am sure it's healthy.
Be sure the vet checks for mange. It's easy to misdiagnose in the early stages, and often mixed up with ringworm. DO keep the cats separate until this one is checked out. Be sure to wash carefully after handling the cat (I'd be wary of tracking the mites in, if it is mange).
Whether it's mange or nutritional (either is likely, if you can give a description of the skin texture and coloration, it would help) the good news is both are very easily treatable. :)
Congrat's on the find of a stray cat! i hear they're hard to come by! :->
It's difficult to say without seeing the cat. Hair loss can be caused by several different things. It can be anything from mange to poor nutrition to flea allergies. As long as the cat is in good general health, all of these things can be easily cured.