Beach water becomes polluted when rainwater washes pollutants (like animal poop, fertilizer, pesticides, and trash) from yards, farms, streets, and construction sites. Pollutants can also come from sewage treatment plants and septic tanks that are not working right.
The pollutants cause germs, or microorganisms, to grow in the water. Microorganisms are tiny living creatures that are too small to see with your eyes, so you can't tell if the water is clean by looking at it. Not all of them are bad, but some can make you sick. Some microorganisms at beaches are bacteria, viruses, worms and protozoa.
Bacteria can lead to infections, diarrhea, and stomach aches. Viruses can cause fever, colds, and intestinal infections. They can also make it hard for us to breathe. Some illnesses caused by worms are coughing, chest pain, fever, vomiting and restlessness. While protozoa can cause intestinal infections, stomach cramps and a serious disease called dysentery.