What is the most common disease in a hot tub?

Pseudomonas Folliculitis

The most common disease transmitted via spas is pseudomonas folliculitis. This is an infection of the hair follicle, caused by Pseudomonas aeruginosa, a bacterium that is present in the environment. Typically, this bacteria is present on the skin, but is in low enough numbers that an infection does not develop. However, in the warm water of the spa, pores open and the bacteria has the opportunity to enter the area around the hair follicle.  If the sanitizer level in the spa water is inadequate, the number of bacteria that enter the hair follicle can overcome the body's natural defenses and cause an infection. This infection is often confused with dermatitis that can occur due to irritation from the spa water. To determine which condition exists, a physician should be consulted.

Maintaining proper sanitizer levels with Chlorine Concentrate or FreshWater Salt System and shocking regularly will minimize the chance of contracting this infection.