Studies on beach sands around Lake Michigan and Lake Superior in the USA have shown that beach sands are an important refuge for E. coli, and concentrations of the bacteria in the sand may be many times greater than that in the water, and persist for considerable periods of time. Scientists estimate that between 10 and 30 people per thousand are in danger of stomach upsets if they eat after playing in the sand without cleaning their hands – simply rinsing reduces the danger dramatically, though scrubbing with soap is best.

Frustratingly these studies refer to fresh water lakes, and it is not clear whether or not the same might be true of sea-water bathing beaches. Still, it is clearly advisable to follow your mother’s advice and always wash your hands before eating!

References
United States Geological Survey (2009, August 12). What Science Says About Beach Sand And Stomach Aches. ScienceDaily. Retrieved August 13, 2009.
American Chemical Society (2007, May 29). Beach Sand May Harbor Disease-causing E. Coli Bacteria. ScienceDaily. Retrieved August 13, 2009.