Scientists have discovered a previously unknown bacterium lurking in human lymph nodes, a finding that suggests there are many more disease-causing bacteria still to be discovered. The bacterium is thought to cause chronic infections in patients with a rare immune disorder called chronic granulomatous disease (CGD), and the research team is now investigating whether it might be involved in conditions that are more common, such as irritable bowel syndrome. Researchers know only a fraction of the bacteria that inhabit the water, air and our bodies, because most of them are impossible to grow and identify in the lab. Even when bacteria are suspected as the cause of a disease, it can be extremely difficult to pin down the exact culprit. The digestive disorder Crohn’s disease, for example, may be partly caused by bacteria. But researchers have been unable to isolate the bugs that are to blame.