The most obvious culprits – like excess weight and a sedentary lifestyle – may not fully explain the increase in colorectal cancer at younger ages. “There may be other factors at play,” says ACS researcher Rebecca Siegel. One possibility: the overuse of antibiotics, which can knock out good-for-you bacteria as well as disease-causing varieties in the digestive system. Some studies suggest that our natural load of bacteria, part of what’s called the microbiome, plays a role in keeping us healthy. “Information about how the gut microbiome influences health in general, and cancer in particular, is still in its infancy,” Siegel says, “but it would make sense that something that changes the microbiome, like antibiotics, might be important.” In some states, enough antibiotics are prescribed each year to dose every inhabitant and still have some left over. Don’t give your doctor the stink-eye when he or she doesn’t prescribe them -- remember: Antibiotics don’t work against colds or other viruses!