Why do extremely obese people develop fungal infections in the folds of their skin?

Intertrigo. Intertrigo is an inflammatory condition of skin folds, induced or aggravated by heat, moisture, maceration, friction, and lack of air circulation. Intertrigo frequently is worsened or colonized by infection, which most commonly is candidal but also may be bacterial, fungal, or viral. Intertrigo commonly affects the axilla, perineum, inframammary creases, and abdominal folds.
Due to moist skin. From overlapping skin due to obesity. Sweat and warmth coupled with overlapping skin makes a perfect petri dish for fungal growth.