Many microorganisms normally live on our skin, including a group of yeast species called Malassezia. (Round or oval yeasts in this group were previously known by the names Pityrosporum orbiculare and Pityrosporum ovalis.) The yeast lives in our pores. Under certain conditions, it can shift its form from a round or oval yeast shape to a string-like, branching shape. These branching forms are named hyphae. The yeast can migrate under the skin and produce azelaic acid, a substance that can change the amount of pigment (color) in new skin cells. In its hyphae form, the yeast causes a rash called tinea versicolor, also called pityriasis versicolor.
Tinea versicolor is common year-round in the tropics and subtropics and is seen in the summer months in more temperate climates. Sun exposure, use of oils on the skin, naturally oily skin and sweating are all suspected to be triggers that can cause the round or oval yeast to convert to its hyphae form, resulting in the rash. This rash can spread between people who have skin contact.
Tinea versicolor occurs more often in teenagers and young adults and appears on the back, neck, upper chest, shoulders, armpits and upper arms. Most people who get this rash are in good health. However, tinea versicolor is more likely to occur if you have a suppressed immune system, for instance, if you are taking a corticosteroid medication such as prednisone for another health problem. It is also more common in women who are taking birth control pills and in pregnant women.