As part of the Canadian Healthy Infant Longitudinal Development (CHILD) study, Finlay and colleagues collected stool and urine samples from more than 300 babies at 3 months and 1 year old, as well as information on their health at 1, 3, and 5 years. Then, they used high-throughput genetic sequencing to detect levels of gut microbes in each stool sample. Babies that had low or undetectable levels of four bacteria—Lachnospira, Veillonella, Faecalibacterium, and Rothia—at 3 months old all went on to show early signs of asthma—wheezing and skin allergies—at a year old. The babies who didn’t develop these symptoms invariably had high levels of the four microbes in their 3-month stool samples.