My sister keeps getting a rash under her armpits and doesn't know what it's from. It's not fungal. She's gotten an antibiotic for it before, but it comes back. She doesn't typically wear any deoderant so I don't think it's a reaction to that. Any idea what she could use to get it to go away? Tea tree oil doesn't help.
Eta: She doesn't shave, use antiperspirant or deoderant.