London - Having siblings, pets and living on a farm reduces the risk of allergic illnesses in babies but having early infections increases it, Danish researchers said on Friday.
Sterile, modern environments have been blamed for the increase in asthma, dermatitis and other allergic diseases over the past century because the immune systems of babies are simply not exposed to many microbes.
Scientists also thought that early infections would have a protective effect against allergies but Christine Stabell Benn, of the Danish Epidemiological Science Centre in Copenhagen, found they increase the risk of developing allergies.
"We found that having older siblings protects against allergic diseases but it is not by means of transferring infectious diseases because those diseases are actually associated with an increased risk of disease in the child," Stabell Benn said in an interview.
"With each infection the risk (of an allergic illness) increases."
The human immune system developed to deal with many different microbes. Scientists believe that when it doesn't encounter them early in life, it overreacts later and allergic diseases develop.
Pets, living on a farm, attending day care and having older siblings increase a baby's exposure to microbes.
Benn and her team interviewed 24 000 women during pregnancy and when their children were six and 18 months old. Their findings are reported online by the British Medical Journal.
About 10 percent of the children suffered from dermatitis at 18 months old. The researchers found that the risk of allergic illness increased with each infectious disease the child suffered before six months old, but it decreased if the child had three or more siblings, attended day care or lived on a farm or with pets.