Changes in food preparation, such as an increase in processed foods and meals, might be a fact – and so might environmental or chemical exposures, such as those from pollution and plastics, says Otis Brawley, a professor of oncology at Johns Hopkins University School of Medicine.

The most common cause of swimming-related illness outbreaks is diarrhea. “This is one of the reasons why, if there is a fecal accident in a pool, there is protocol the lifeguards and pool operators follow before allowing people back into the water,” explains Dr. Clare Rock, an associate professor at the Johns Hopkins University School of Medicine.