What could be the cause of our cats hair loss?

Recently, my cousin gave my family a cat. She had hair loss problems, and we thought it was stress or diet issues. But now, one of our other cats is losing his fur, the same way. The hair loss is on the back of the legs, the lower back and abdomen and the tail. It's particularly bad at the base of the tail. ... show more Recently, my cousin gave my family a cat. She had hair loss problems, and we thought it was stress or diet issues. But now, one of our other cats is losing his fur, the same way.

The hair loss is on the back of the legs, the lower back and abdomen and the tail. It's particularly bad at the base of the tail. They have both been grooming themselves a lot lately.

What could the problem be? Is there a cure? Will the hair loss be permanent(he's such a beautiful cat-- long haired flame point siamese)?
Update: It's not fleas and it's not in rings, it's kind of gradual, like that cat is grooming himself bald. But only his lower half.
Update 2: And they are both indoor cats.
Update 3: We've had fleas before, and it wasn't an issue, meaning he didn't lose his fur. We have been keeping them up to date with flea medicine and the new cat has been too. We've checked for fleas, and fleas are pretty visible. Then we get this new cat who is losing her fur, on a certain part of her... show more We've had fleas before, and it wasn't an issue, meaning he didn't lose his fur. We have been keeping them up to date with flea medicine and the new cat has been too. We've checked for fleas, and fleas are pretty visible.

Then we get this new cat who is losing her fur, on a certain part of her body. Whatever it is spreads to another cat, losing his fur in the same pattern. It sounds more like a disease or a fungus than fleas.
Update 4: Food alergies are not contagious. Whatever it is spread from one cat to another cat. It is not fleas, we have checked for that. If anyone knows what it is, can you tell me if the treatment is expensive, because since both my sister and I are headed off to college this year it's getting harder to find the... show more Food alergies are not contagious. Whatever it is spread from one cat to another cat.

It is not fleas, we have checked for that.

If anyone knows what it is, can you tell me if the treatment is expensive, because since both my sister and I are headed off to college this year it's getting harder to find the money to even take the cats to the vet.

Please help, I'm very stressed out about this.
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