When it comes to disease mitigation, cryptosporidium is a tough bug to kill.

Cryptosporidium is a waterborne protozoon that causes flu-like symptoms that can last for two to four weeks. Symptoms include nausea, stomach cramps and pain, vomiting, fever, weight loss, and diarrhea.

The sickness is generally spread through drinking water and swimming pools.

Cryptosporidium is introduced to pools by swimmers with diarrhea.

One of the problems with cryptosporidium is that it is highly resistant to traditional chlorine disinfection methods. When chlorine is added to a pool, it has varying contact time values to inactivate different pathogens. For cryptosporidium, that time is almost 7 days. That means that cryptosporidium can lurk in the pool for a full week, almost totally unaffected by the chlorine.

The sickness is passed when swimmers ingest the water, an inadvertent, but normal swimming act. The EPA has found that adult swimmers typically swallow 24 milliliters (a little less than an ounce) of water per swim period, while children consume twice that amount. In a pool contaminated with cryptosporidium, the danger is obvious.

There are a number of ways to reduce the incidence of the pathogen in swimming pools. The most obvious, of course is not to enter the pool while experiencing diarrhea, as well as showering prior to entering the water.

But sometimes accidents happen, and the CDC has strict requirements about what to do should a diarrhea incident happen at any aquatic venue.

As stated above, Cryptosporidium is resistant to chemical treatment — even a lot of chlorine — especially if appreciable levels of cyanuric acid are in the water.

The CDC has conducted studies on the effect of cyanuric acid on the kill rates of cryptosporidium. They were unable to achieve a 3-log reduction (99.9 percent inactivation) of cryptosporidium when cyanuric acid levels were above 16 ppm. For that reason, the pool must be drained and diluted until the water contains less than 15 ppm cyanuric acid. Then, at least 10 ppm chlorine must be added and maintained for about 25 hours. The concentration of chlorine needed and time maintained may be calculated using CT inactivation values. See graphic above.