Moms can relax. Dirty little children aren’t such a bad thing.

Exposure to germs as a kid seems to be helpful, while living in an environment that’s squeaky clean seems to pose risks for some illnesses. Still, nobody knew precisely why. But now some scientists say they think they’ve figured out the details of the “hygiene hypothesis.”

They found that microbes in the gut keep a rare part of the immune system reined in. No microbes, and the immune cells go crazy in the lungs and intestines, increasing the risk of asthma and colitis. Add in the microbes, and cells in question, invariant natural killer T cells, retreat.

Aside from toxic waste, getting grimy and being exposed to a variety of germs in the environment can actually improve immunity.

Women have stronger immune systems than men

Maybe that’s why women live longer than man.

Women are known to have a lower incidence of cancer — men have a two- to five-fold greater risk of developing the disease. Women are also better able to survive trauma, and, according to some reports, don’t get as seriously ill from bacterial and viral infections.

In a new paper, researchers from Ghent University in Belgium argue these sex-specific health disparities may be due, at least in part, to tiny pieces of genetic material called microRNAs. The main function of microRNAs in cells is to turn off, or “silence,” specific genes. The researchers say microRNAs located on the female X chromosome may give women an immune system advantage over males.