Beach sands can harbor a slew of stomach bugs, which can cause bouts of nausea and cramping or even severe gastroenteritis. A 2009 study in the American Journal of Epidemiology found that digging in beach sand (and being buried in beach sand) raised the risk of diarrhea. Beachgoers could come down with some familiar pathogens, such as Salmonella, E. coli and norovirus—the most common stomach flu in the U.S., seen here. Parasites such as Giardia and Campylobacter are also on the watch list.
The prevalence of stomach bugs at the beach comes down to one thing: poop. Traces of fecal contaminants and microbes have been found in sand samples and in some water samples at both freshwater and marine beaches. How does poop end up at the beach? Some studies point to human sewage contamination from nearby treatment plants. But fecal pollution also shows up at beaches far from treatment facilities. Storm surges and heavy rainfall could carry human waste further from a source, or wild animals may contribute their droppings.
Keep in mind, seawater that renders 36 out of 1000 swimmers sick with gastroenteritis is acceptably clean by EPA standards.