The average person's body contains about 100 trillion cells, but only maybe one in 10 is human. Human cells ... are far outnumbered by those from microbes - primarily bacteria but also viruses, fungi and a panoply of other microorganisms. That thought might make a lot of people lunge for the hand sanitizer, but that impulse may be exactly the wrong one. Researchers are amassing a growing body of evidence indicating that microbial ecosystems play crucial roles in keeping us healthy. Moreover, scientists are becoming more convinced that modern trends - diet, antibiotics, obsession with cleanliness, Caesarean deliveries - are disrupting this delicate balance, contributing to some of the most perplexing ailments, including asthma, allergies, obesity, diabetes, autoimmune diseases, cancer and perhaps even autism. These microbial stowaways may wield far greater powers than previously appreciated. Acquired beginning at birth, this mass of fellow travelers may help steer normal development. Investigators are trying to identify which organisms may truly be beneficial probiotics" that people could take to help their health. One intriguing finding is that babies born through Caesarean section apparently miss out on acquiring their mothers' microbiota. The rising number of C-section babies ... might help explain trends such as rising incidents of asthma and food allergies caused by misfiring immune systems. Obese people appear to have a distinctive mix of digestive bacteria that make them prone to weight gain. Thin mice get fatter when their microbiota is replaced with the microbes of obese animals.