Pseudomonas aeruginosa folliculitis is the most common recognizable infectious disease occurring after use of whirlpools and hot tubs. The factors that affect the host's susceptibility to whirlpool-related infection are the anatomic and physiologic defenses of normal skin, the microecology of the skin surface, factors intrinsic to the individual host, and behavioral factors. The structural components of the skin maintain an environment at the skin surface that makes human skin an inhospitable habitat for microflora. However, natural and experimental models of P. aeruginosa skin infection suggest that immersion in whirlpools may negate many of the body's normal host defenses, especially the very low humidity at the skin surface. Transient colonization of skin with P. aeruginosa may lead to elaboration of toxins in vivo, resulting in the characteristic dermatitis.