In the 1950s, a series of epidemiological studies were done in the US to determine the relationship between water quality of natural waters and the health of bathers. The results indicated that swimmers were more likely to have gastrointestinal symptoms, eye infections, skin complaints, ear, nose, and throat infections and respiratory illness than non-swimmers and in some cases, higher coliform levels correlated to higher incidence of gastrointestinal illness, although the sample sizes in these studies were small.  Since then, studies have been done to confirm causative relations between swimming and certain health outcomes.  A review of 22 studies in 1998  confirmed that the health risks for swimmers increased as the number of indicator bacteria increased in recreational waters and that E. coli and enterococci concentrations correlated best with health outcomes among all the indicators studied.  The relative risk (RR) of illness for swimmers in polluted freshwater versus swimmers in unpolluted water was between 1–2 for the majority of the data sets reviewed.  The same study concluded that bacterial indicators were not well correlated to virus concentrations.