What causes fungal acne?

The yeast: always there, sometimes too much

Malassezia is part of the normal skin flora. The European Academy of Dermatology and Venereology describes Malassezia folliculitis as what happens when the harmless colonisation of hair follicles by these yeasts becomes symptomatic, with itchy papules and pustules (Henning and colleagues, 2023). A 2014 review puts it the same way: an overgrowth of yeast present in the normal flora, often misdiagnosed as ordinary acne (Rubenstein and Malerich, 2014).

So the question is less where the yeast came from and more what let it grow. Malassezia cannot make its own fatty acids. Genome studies found the gene for that missing in every species (Wu and colleagues, 2015), so it takes fats from its surroundings. On skin, the main supply is sebum, the oil the skin makes, which the yeast breaks down (Ro and Dawson, 2005). That is why oily areas such as the upper back, chest and forehead are where it shows up.

The triggers doctors have recorded

These come from case series and clinical references. None of them is a controlled trial, and most people with one or more of these never develop it.

• Antibiotics. In a review of 110 patients aged up to 21 with a confirmed diagnosis, more than 75 percent had acne recently treated with antibiotics (Prindaville and colleagues, 2018). The 2014 review also names antibiotic use. The authors of the 2018 paper noted that, unlike ordinary acne, this condition was more common after antibiotics.
• Heat, humidity and sweat. DermNet lists a hot humid climate and excessive sweating as risk factors (DermNet).
• Oily skin. High sebum production is on DermNet's list, and a 1985 series of 51 patients named greasy skin as a probable predisposing factor (Bäck and colleagues, 1985).
• Occlusion. The 1985 series named occlusion too, and DermNet lists occlusion from emollients and sunscreens. Tight synthetic clothing over sweaty skin is the everyday version.
• A weakened immune system. Immunosuppression, including steroid use, is named by DermNet and by the 2014 review.

Where skincare fits

Heavy creams and oils come into the occlusion trigger above. Separately, lab studies have shown which kinds of ingredient Malassezia can use as food: certain fatty acids, their esters, polysorbates and many plant oils. The fungal acne checker explains each class and the study it comes from.

What nobody has shown is that avoiding those ingredients prevents fungal acne or clears it. The skin's own sebum feeds the yeast whatever you put on it, so product changes are at most one part of the picture.

What it looks like, and how it differs from acne in the same places, is covered on fungal acne on the forehead and fungal acne vs hormonal acne. This site shows no clinical photographs. DermNet carries licensed ones on its Malassezia folliculitis page.