For all their vital function, sweat glands have an inherent weakness: Their tiny ducts can be prone to clogging. If this happens, the sweat that would normally exit the body gets trapped in the gland, giving infection-causing bacteria a safe haven to grow and multiply. While infections in both eccrine and apocrine sweat glands involve clogged ducts, the root causes of infection vary by gland type.
Miliaria, an infection of the eccrine sweat glands, can be triggered by excessive sweating or too much time spent in a hot, humid environment. Sweat may build up in blocked eccrine glands to form miliaria crystalline, a rash of tiny, painless vesicles. If infection ruptures the gland's ducts, sweat can move into the skin around the glands, causing an itchy inflammation called miliaria rubra, or "prickly heat." If this condition advances, the infected glands can swell with pus in a condition known as miliaria pustulosa. And in miliaria profunda, the infection's most advanced form, sweat leaks into the surrounding dermis, causing a severe burning sensation [sources: Baker, New Zealand, O'Connor].
Because they affect hair follicles, infections of the apocrine glands are often referred to as folliculitis. Unlike miliaria, these infections can occur regardless of the amount that someone sweats. The exact disease triggers aren't always known, but people with certain complicating conditions can be more prone to folliculitis [sources: Bakr]. Most types of folliculitis involve keratin or other bodily substances plugging the apocrine gland's duct where it opens to the hair follicle, leading to infection in the gland. While some types of folliculitis may be due to genetic predisposition -- the body fails to shed skin and hair cells properly, leading to plugged ducts -- other types of the infection can be brought on by poor hygiene, such as shaving the armpits or groin with a dull, dirty razor, or chafing from tight-fitting clothing [source: Jovanovic].
One of the most advanced and uncomfortable forms of apocrine gland infection is hidradenitis suppurativa, also known as acne inversa. The condition appears as blackheads and painful, sometimes pustulent bumps in the groin and armpits. It can persist for years, and often gets worse as the victim ages. Chronic bouts of hidradenitis suppurativa can form networks of scars just below the skin's surface [source: Jovanovic].