There are four main blood groups: A, B, AB, and O (all of which can be positive or negative), determined by the genes bequeathed to you by your parents. Only one of them, however, makes you more vulnerable to a severe infection by a strain of Escherichia coli that causes illness, including the rather ghastly traveler’s diarrhea (TD).

Sorry, Type As. Best to practice the best hygiene possible as you trot around the globe, lest you get a fountain of feculent fondue sprouting from your lower orifice.

TD is infrequently dangerous to those in the developed world, but it’s decidedly grim. Its somewhat colloquial name stems from the fact that it tends to be picked up in parts of the world where sanitary practices aren’t exactly tip-top or where the climate is more microbe-friendly, so to speak.

Although the symptoms can vary from person to person, it’s generally defined as experiencing three or more loose/water bowel motions in 24 hours and is occasionally accompanied by abdominal cramps, the sudden urge to let loose from below, nausea, and vomiting.

The Washington University School of Medicine-spearheaded research group noted that children in Bangladesh with blood type A were more vulnerable to serious diarrheal infection than others. In order to find out why, as is nonchalantly elucidated in The Journal of Clinical Investigation, volunteers at Johns Hopkins University with numerous blood types were “challenged” with a type of E. coli that produces a toxin that affected the intestines.

Although not the only cause, these nasty E. coli – one of very few to be harmful – are known to be “a principal cause of diarrhea in travelers”. More seriously, these microbes are also responsible for hundreds of thousands of deaths in the developing world, particularly of young children.

The strain, H10407, was isolated from a case of “cholera-like” illness, before being given to the brave, brave participants to wolf down, like the world’s worst shot. As the team explain, “blood group A human volunteers developed severe diarrhea more frequently than individuals from other blood groups.”

Clearly, everyone had a nightmarish time, but those particular individuals had the worst of it; they got sicker faster too.

It appears that this strain of E. coli produces a protein that preferentially attaches itself onto A blood group flavors of molecules named glycans, which are expressed on the surfaces of cells within the small intestine. This allows the malevolent critters to enhance their toxin delivery, which makes these unfortunate few more severely ill.

Ultimately, then, those volunteers didn’t break their bowels for nothing: This discovery means that, weaponized with this knowledge, bespoke vaccines could protect A blood group people – from Instagram-prone travelers to children most at risk in the developing world – from this highly unwelcome fate.

This protein is not found in all strains of dangerous E. coli, though; it’s common, but not ubiquitous. Expect more hardy souls to volunteer their poor small intestines up for the sake of science in the near future, then, before a "universal" vaccine emerges.

The team are also keen to emphasize this shouldn’t affect how we travel. Regardless of whether or not you have type A blood – and yes, that (semi) includes you AB types – you should always practice good hygiene when you travel.