Children growing up in rural areas, around animals and in larger families seem to develop asthma less often than do other children. According to the hygiene hypothesis, this is due to increased exposure to particular viruses, bacteria or parasites.

The hygiene hypothesis proposes that childhood exposure to germs and certain infections helps the immune system develop. This teaches the body to differentiate harmless substances from the harmful substances that trigger asthma. In theory, exposure to certain germs teaches the immune system not to overreact.

But preventing asthma isn't as simple as avoiding antibacterial soap, having a big family or spending time on the farm. Current research suggests that this potentially helpful exposure to certain germs may actually occur before a baby is born, when he or she is still in the womb. Researchers are discovering that a pregnant mother's exposure to infectious germs or other substances, such as vaccines, may play an important role in the development of a baby's immune system and gut microbiome.

For now, more research is needed to understand exactly how childhood germ exposure might help prevent asthma.