The bacterium, called Helicobacter pylori, 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, but how infection is related to diarrheal diseases and the bacteria that cause them is still up for debate.
Dani Cohen, the lead author of the new report from Tel Aviv University in Israel, said that some previous studies have suggested that being infected with the ulcer-causing bacteria increases the risk of diarrhea, while others have reported finding the opposite.
So his team carried out a study on 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 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% of soldiers who had diarrhea due to different types of bacteria or to an unknown cause had been infected with H. pylori before training, compared to up to 56% of soldiers who never reported diarrhea.
The researchers calculated that being infected with H. pylori meant soldiers were about 60% less likely to get diarrhea from Shigella bacteria or other unknown causes.
Carrying H. pylori was also linked to a lower chance of having diarrhea caused by Escherichia coli bacteria – but statistically, that particular finding could have been due to chance.
Cohen explained in an email that an H. pylori infection may affect how acidic the gut is, and high acidity is known to keep disease-causing bacteria from settling there.
The researchers also wrote in the journal Clinical Infectious Diseases that having an immune system in overdrive due to chronic H. pylori infection may also keep other bacteria in the digestive system at bay.
Since the discovery of H. pylori and its role in stomach cancer, doctors and scientists have debated the merits of using antibiotics to eliminate the bacterium from the stomachs of people who carry it.
"This is another piece of evidence on the various influences on health that H. pylori has, as has been reported in the medical literature," Cohen told Reuters Health.
While the negative health effects include ulcer disease, stomach cancer and low iron levels, he said, the potential benefits seem to be a reduced risk of asthma and esophagus cancer, and now diarrheal diseases.