Scientist may have uncovered the reason why the body cannot eradicate a potentially dangerous stomach infection.
H.pylori causes stomach ulcers
Their work on the H.pylori bacterium could pave the way for better treatments aimed at curing the problem.
H.pylori, which causes stomach ulcers and is linked to gastric cancer, seems to be able to evade the immune system.
Research from Germany suggests that a chemical it secretes interferes with the immune cells sent to destroy it.
Half the population
H.pylori - short for Helicobacter pylori - can be found in the guts of half of all middle-aged men.
Most never fall ill as a result, but a significant minority develop stomach ulcers, which are painful and can be dangerous in themselves.
This problem has been connected with an increased risk of getting stomach cancer later in life.
The vast majority of serious bacterial infections operate on a "kill or be killed basis" - they are either wiped out by the immune system within a short period, or they overwhelm their host.
H.pylori is not like this, and has the ability to become a persistent infection, able to resist the attentions of the immune system for years, or even decades.
Doctors have the option of using existing powerful antibiotic treatments to tackle the infection - but many are reluctant to treat patients this way, especially if they have the bug, but no symptoms.
Experts believe that the research, from the Max von Pettenkofer-Institute fur Hygiene und Medizinische Mikrobiologie in Munich, could eventually help produce better ways of treating the infection.
They have discovered more about how a toxin released by H.pylori is able to target a chemical signalling pathway used by the immune system.
Ultimately, this disrupts a mechanism which produces inflammation - a key immune response.
Professor Charles Penn, from the University of Birmingham, told BBC News Online that the find could be important.
"There is some thought that not all H.pylori is bad for you, so if you use treatments that eradicate 'the good guys', you might open the door for infection with the 'bad guys'.
"Research might mean you could target only the 'bad' H.pylori."