Researchers measured bacterial levels in seawater at four beaches in Northern Spain three times a week from July to mid-September in 1998, and interviewed 2000 bathers, with nearly all being contacted again seven days later. Symptoms of infection were reported by 7.5% of the respondents, which correlated with total numbers of intestinal and faecal bacteria found at the sites. Visitors were found to report more symptoms than residents, and swimmers were 10 times as vulnerable as non-swimmers.
According to the researchers, almost 40% of water samples exceeded the guide standard for total coliforms, while 57% and 38% of samples exceeded the guide standard for faecal coliforms and other faecal bacteria, respectively.