Top: Manifestation in 18-year-old male
Bottom: Causative yeast cells
Unlike the other skin mycoses (called tinea - from Latin, “meaning a gnawing moth, or bookworm”), tinea versicolor is not caused by the same dermatophytes of the genus Ascomycota that most “ringworm”-type afflictions belong to.
Tinea versicolor is a yeast, which is a fungus of the phylum Basidiomycota, and a fairly common condition. Between 2 and 8% of people in the United States express this condition, especially during the summer months, as tinea versicolor thrives in hot and humid conditions when there is excess skin oil and dead skin cells to consume. Among young adults and adolescents, this condition affects almost 10% of individuals.
These yeasts (Malassezia furfur and Malassezia globosa) are not considered harmful, and are generally asymptomatic aside from its dermatological manifestation. When the condition recurs during multiple summers, the source of the yeast is generally high environmental concentrations of the spores; unlike many vegetable or animal parasites, the yeasts that cause tinea versicolor are not known to lay dormant on humans for more than a few weeks.
The original cures for this condition included both oral and topical mercury treatments - needless to say, topical mercury did kill the fungus, but you can’t exactly justify the harm done, just to clear up a harmless condition that isn’t all that disfiguring to begin with. These days, tolnaftate (brand name Tinaderm) and selenium sulfide (like in Selsun Blue shampoo) are used to treat the condition, and can generally clear up the condition. However, when the source is due to high environment concentrations, it can often recur, especially while one is maturing, and skin oil/dead skin cells are more prevalent than usual.
Treatise on Diseases of the Skin, for the use of Advanced Students and Practitioners. Henry W. Stelwagon, 1914.