The Thames has a reputation as a dirty river. It’s not totally undeserved. I took part in the notorious Hampton Court to Kingston swim in October 2012, when I was one of 338 people who reported getting ill. No specific cause was identified. It was most likely a bacterial or viral infection. In the days prior to the race, rainfall was several times the average for the time of year. The river may therefore have been contaminated with sewage from Combined Sewer Overflows (CSOs), that can spill into the river during periods of heavy rain.