Most infections from swimming pools and hot tubs are caused by cryptosporidium, a diarrheal disease caused by microscopic parasites which are released (anally) into the water and then consumed (orally) by fellow swimmers. Hotel pools are the primary place of infection. – WTF Fun Facts
Source: https://www.vice.com/en_us/article/4354ed/how-gross-is-the-hot-tub-in-my-hotel-or-spa