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You seem to be having Trichomycosis axillaris. It is a bacterial infection of hair over the under arms. The best treatment will be to shave off the underarm hair. You may use a benzoyl peroxide
2.5 % soap or wash and apply fusidic acid
cream thrice a day for cure. Get these prescribed by your primary care
physician. Using antiperspirant will help prevent recurrence.
Hope this helped