Swimming in contaminated pools can cause gastroenteritis from water contaminated by viruses, bacteria, or parasites. Germs that cause gastroenteritis are shed in feces of infected persons, and easily spread to uninfected persons swimming in pools. Symptoms of gastrointestinal illness can include nausea, vomiting, watery or bloody diarrhea, and weight loss. Common causes of swimming-related gastroenteritis included viruses (norovirus), parasites (giardia, cryptosporidium), and bacteria (Escherichia coli, Shigella).