The chance of a swimmer in the U.S. being attacked by a shark is close to zero.
But dangerous creatures do lurk in our waters, including lakes, ponds and even concrete swimming pools.
Microbes such as cryptosporidium, shigella, giardia and E. coli frequently live in public swimming areas. All three cause diarrhea if ingested.
"Accidently swallowing contaminated water, or even getting it in your mouth, can make you ill," said