Infants exposed to a variety of household bacteria carried through rodent and pet dander and, yes, even roach droppings, are less likely to suffer from allergies, wheezing and asthma, according to a new study.

Research through the Johns Hopkins Children's Center in Baltimore, along with other institutions, and published in the Journal of Allergy and Clinical Immunology, shows early exposure to bacteria and certain allergens may end up protecting a child by building up the immune system.

Prior studies have shown children who grow up on farms have lower allergy and asthma rates, which scientists attribute to their regular exposure to microorganisms plentiful in farm soil.

On the other hand, or antenna, as it were, other research has suggested the risk of suffering asthma increases among inner-city dwellers exposed to high levels of roach and mouse allergens and pollutants.

Indeed, the new study confirms children who live in homes with rodents and insect infestations do show higher overall allergy and asthma rates; if, however, a kid encounters such such substances before his or her first birthday, the body seems to benefit from the exposure, rather than be hurt by it.

Yet, the protective effects of both allergen and bacterial exposure were evident if a child's first encounter with the substances happened after age 1, the research found.

"Our study shows that the timing of initial exposure may be critical," study author Robert Wood, chief of the Division of Allergy and Immunology at Johns Hopkins Children's Center, said in a news release. "What this tells us is that not only are many of our immune responses shaped in the first year of life, but also that certain bacteria and allergens play an important role in stimulating and training the immune system to behave a certain way."

The study was conducted among 467 inner-city newborns -- from Baltimore, Boston, New York and St. Louis -- whose health was tracked over three years.

Investigators visited homes to measure the levels and identify the types of allergens present in the infants' surroundings.

The researchers also tested the infants for allergies and wheezing, with periodic blood and skin-prick tests, physical exams and parental surveys.

The researchers also collected and analyzed the bacterial content of dust collected from the homes of 104 of the 467 infants in the study.

When researchers studied the effects of cumulative exposure to both bacteria and mouse, cockroach and cat allergens, they noticed another striking difference: children free of wheezing and allergies at age 3 had grown up with the highest levels of household allergens and were the most likely to live in houses with the fullest array of bacterial species.

In other words: about 41 percent of allergy-free and wheeze-free children had grown up in allergen and bacteria-rich homes, while only 8 percent of children hindered by both allergy and wheezing had been exposed to these substances in their first year of life.

According to the United States Centers for Disease Control and Prevention, asthma is one of the country's most common pediatric illnesses, affecting an estimated 7 million children

Further, by the time they turn 3 years old, up to half of all children develop wheezing, which often evolves into full-blown asthma.