hair loss?

OK so the rescue I work at had 2 horses in 2 separate pastures come up with bald spots literally overnight. They are on their necks, face, and chest with a few on their butts. When you mess with the hair around the bald spots it falls out with lots of white flakey skin like stuff. We called the vet and she thinks its a lack of a certain mineral. So we bought her loose minerals and filled all the tubs. Now 2-3 others have small bald spots developing on thier butts. What could it be? They have only been on the mineral a day now and the vet wants to try a week at least (I know it will take time) but I've never seen anything like this. I don't have pics but the skin isnt red or irritated, it looks like normal skin. The hair falls out just by touching it. Its creepy when you can pinch a large section of hair and it falls out in your hand. Im just curious what could be wrong with them? If it is indeed just a mineral thing, how long before we see effects, what mineral are they lacking? I wasnt the one who talked to the vet so I couldnt ask her all these questions. Im the type of person who likes to be fully educated :)
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All my research says rain rot which is a lack of vitamin A. Can anyone explain more? Such as how much vitamin A they need, good vitamin A supplements, what exactly is rain rot (a fungus, a deficiency, both), best treatment options, ways to make the hair grow back, hiw libg before it goes away, what can we do to prevent it in the future, is it contagious? Im confused because all of my research seems to contradict itself. I always thought it was a fungus that was treated topically. When my gelding caught a small spot of it, we treated it with an OTC spray, it cleared up in a few days and that was that. These horses seem to come up with huge bald patches literally overnight.
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