The researchers show that in mice, exposure to microbes in early life can reduce the body's inventory of invariant natural killer T (iNKT) cells, which help to fight infection but can also turn on the body, causing a range of disorders such as asthma or inflammatory bowel disease (<a href="http://www.sciencemag.org/content/early/2012/03/21/science.1219328">abstract</a>).