A type of acne called pityrosporum folliculitis is caused by a yeast, which sweat encourages. You usually find it across the upper chest and back, characterized by uniform, pinhead-size pimples and little white bumps that may itch. Breakouts most often occur in hot and humid environments or on people who exercise intensely or perform hard manual labor. Pityrosporum folliculitis is usually resistant to antibiotics and requires treatment that addresses the yeast overgrowth.
Pityrosporum folliculitis results from an overgrowth of a naturally occurring yeast that you have on your skin. Excess sweat can encourage the yeast to grow in the pores -- especially if you wear synthetic clothing that's tight and retains moisture. Greasy sunscreen, especially when combined with sweat, can also foster yeast overgrowth. You may be more vulnerable if your immune system is weak or you're under a lot of stress. Some people are more likely to develop the condition; researchers aren't sure why.