What is the main cause of boils on the skin under the arm?

In most cases, boils under the arm occur when Staphylococcus aureus bacteria enter the skin via minor abrasions or bug bites and create an infection, according to Mayo Clinic. As the infection progresses, the underlying hair follicles become inflamed.

Individuals with medical conditions such as diabetes and those who practice poor hygiene habits are at higher risk for developing boils, explains WebMD. Additional risk factors include inadequate nutrition, chemical irritation of the skin and immune system disorders. Individuals with underarm boils should seek immediate medical attention if they experience symptoms of infection, such as fever, swollen glands and severe pain.