The womb is a (mostly) sterile environment. When babies leave it, they are thrust into a world that’s positively teeming with microbes—bacteria, viruses, fungi, parasites, and more. Some threaten to infect them and cause disease. Others are necessary for their survival and colonise their guts, skin and other organs.

Babies need to keep the dangerous microbes out, and let the beneficial ones in. It’s a tough balancing act, and one whose solution has only just been discovered.

Shokrollah Elahi from the Cincinnati Children’s Hospital Medical Center has found that newborn mice have special immune cells that suppress the rest of their immune system. This makes them unusually vulnerable to infections for the first two weeks of life, but it also