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. For one thing, there are a number of microbes — such as respiratory syncytial virus (RSV) — that may cause asthma rather than prevent it. In addition, infections that might help prevent asthma can cause a number of other health problems. The type of germ isn't the only factor that plays a role, either. The severity of an infection and when the infection occurs during childhood also appear to matter.
More research is needed to understand exactly how childhood germ exposure might help prevent asthma. What we do know is that in children with asthma, exposure to germs is likely to do more harm than good.