Cryptosporidium

Source: Water can become contaminated when in contact with animal waste, runoff from a farm with animal waste, or when it contains animals infected with Cryptosporidium.

Treatment: https://tdb.epa.gov/tdb/contaminant?id=253534853

Summary: Cryptosporidium is a resilient single-celled parasite (Protozoa) that may result in an intestinal infection and diarrhea complications. It is spread through fecal matter and can be contracted by ingesting contaminated water, contaminated food, or improper sanitary habits after coming in contact with excrement. In the infectious stage of the Cryptosporidium’s life cycle it is called an oocyst. This oocyst is resistant to chlorine, too small to be caught by many filters, and in proper conditions can survive for up to 18 months in nature. Cryptosporidium will reproduce within the digestive tract, and can produce anywhere from 1-10 billion oocytes a day.