In Sweden in the past decade, there have been six outbreaks of waterborne campylobacteriosis, which causes gastroenteritis. In fact, a total of 27 000 people suffered from waterborne disease in Sweden in those ten years. During the same period, the United Kingdom reported 13 outbreaks of cryptosporidiosis, which also causes gastroenteritis and can usually be traced back to agricultural pollution.
Most swimming-related diarrhoea is caused by infective agents of the genus [Cryptosporidium]. Unlike bacteria, the parasites are extremely resistant to chlorine and may remain infective for several days in swimming pool water containing recommended chlorine concentrations. A single infected faecal discharge in a swimming pool is potent enough that a few mouthfuls water can result in infection.