Asthma is less common in rural areas in Africa than in towns.
In this week's issue of the Lancet, Professor John Britton and colleagues report results from a study carried out in Jimma, Ethiopia.
A total of 9844 people living in towns and 3032 people living in the country were involved.
The researchers asked the participants whether they wheezed or not over the previous 12 months. They also took samples of their feces to test for hookworm.
Those people whose fecal samples tested positive for hookworms were half as likely to have asthma as those who tested negative for hookworm.
| Individuals with hookworms were half as likely to have asthma.
The authors suggest that hookworms in the intestine may alter the way the body reacts to the factors that may provoke asthma.
The authors state, "Parasites locally suppress the host's immune system. This phenomenon has probably evolved to facilitate the passage of the parasite through host tissues.
"The study's findings support the notion that this might also suppress local inflammatory responses to allergens."