Seal armpit brings discomfort, aches. To delay visit doctors should not. Formation under the muscle cavity may turn out to be cancer disease. But more often it is consequence frequent use antiperspirants, non-compliance rules personal hygiene, strong sweating. Redness causes narrow blouse, alien razor, the infection. 
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Discomfort under the arm, inflammation, lump becoming warm? This is the boil that should cut, then drink antibiotics. first stage treat solutions, antibacterial drugs. If the process is started, need surgical procedure.