That news appears in the Archives of Internal Medicine.
The bacterium is called Helicobacter pylori (H. pylori). It is associated with ulcers and increased risk of stomach cancer.
H. pylori is found worldwide, but it's more common in developing countries, note Yu Chen, PhD, MPH, and Martin Blaser, MD.
They studied data on 7,663 U.S. adults who took part in a national health study conducted from 1988-1994.
Participants were asked if they had ever had asthma. They also took a blood test to see if they had ever been infected with H. pylori. A subgroup of nearly 2,400 participants also took skin tests to check their skin's sensitivity to pollen and molds.
H. Pylori and Asthma
Overall H. pylori infection didn't affect participants' odds of currently having asthma. But it apparently reduced their risk of having had asthma in the past, especially in childhood.
"The present observations are consistent with the 'hygiene hypothesis' that microbial infections during early childhood may prevent or diminish [skin] sensitization and asthma," write the researchers.
H. pylori may have "costs and benefits," Blaser says in a New York University news release. "The relative costs and benefits clearly differ among individuals," he adds.
The study doesn't show how H. pylori makes asthma less likely.
"One hypothesis is if you have H. pylori in your stomach, you have an inflammatory process that is ongoing for decades, and this is skewing the immune system in a particular direction," Blaser says.