Some previous studies had suggested that being infected with the bacterium, Helicobacter pylori, increases the risk of diarrhea, while others have reported finding the opposite, said researchers from Tel Aviv University.

"Our findings suggest an active role of H. pylori in the protection against diarrheal diseases," wrote lead author Dani Cohen in the journal Clinical Infectious Diseases.

The bacterium is especially common throughout the developing world, but only causes symptoms in a minority of those it infects. People with chronic H. pylori infections are known to have an increased risk of stomach cancer and related diseases.

Cohen's team studied 595 male Israeli soldiers, close to one-third of whom visited a base clinic for diarrhea during their field training.

All of the soldiers had their blood taken before the start of training, which researchers used to determine which men were chronically infected with H. pylori.

It turned out that between 32 and 36 percent of soldiers who had diarrhea due to different types of bacteria than H. pylori, or from unknown causes, had been infected with H. pylori before training.

By contrast, up to 56 percent of soldiers who had been infected with H. pylori before training reported no diarrhea.

The researchers calculated that being infected with H. pylori meant solders were about 60 percent less likely to get diarrhea from Shigella bacteria.

They also had a lower chance of having diarrhea caused by Escherichia coli bacteria -- although statistically, that particular finding could be due to chance.

An H. Pylori infection may affect how acidic the gut is, and high acidity is known to keep disease-causing bacteria from settling there, Cohen said in an email.

Having an immune system in overdrive due to chronic H. pylori infection may also keep other bacteria in the digestive system at bay, the researchers wrote.

SOURCE: bit.ly/sE7yOd

(Reporting from New York by Genevra Pittman at Reuters health; editing by Elaine Lies; Editing by Idayu Suparto)

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