Chapter 12 399 Work on Malassezia physiology will provide insight into the mechanisms by which basidiomycete fungi have adapted to the mammalian skin environment. This research will also provide new opportunities to dissect specific interactions between commensal fungi and skin. A deeper understanding of these interactions may lead to new treatment paradigms and ways to intervene in the effects of Malassezia on human and animal health. Hopefully, new, more fundamental understanding of the interactions between Malassezia and human skin will enable development of new tools to manage both the number and the activity of these unique fungi. Prevalence Dandruff is the most common scalp disorder in adolescence (post-pubescence) and adulthood, but is rare and mild in children. 51-53 Historically, it was thought that about 50% of humans were affected to some degree, with onset at puberty and peak incidence and severity at about 20 years of age and becoming less frequent after the age of 50.51-52 A more recent study of 1,408 Caucasians, African Americans and Chinese from the states of Minnesota and Georgia in the United States, Beijing, Shanghai, and Guangzhou, China, suggests that severity and prevalence of noticeable dandruff and seborrheic dermatitis is much higher in adults than first thought, at 81‑95% in African Americans, 66‑82% in Caucasians, and 30‑42% in Chinese (see Table 1) 6 . Additionally, the prevalence of dandruff was as high in US teens as their adult counterparts with prevalence at 75‑95% in Caucasian and African American teens. 5,54 Based on this survey, dandruff occurs in 60‑90% and seborrheic dermatitis in 3‑5% of immunocompetent adults. In AIDS patients, the prevalence of seborrheic dermatitis increases to 30‑33%. 55 Dandruff does not seem to vary with climate, as incidence and severity are similar from regions north to regions south in both the United States and China. 5,54 As may be predicted, more frequent shampooing results in lower severity in all populations,5 but the use of specific antidandruff products must be considered. Despite