Asthma is a long-term lung disease characterized by inflamed and narrow airways. About 18.7 million people in the US suffer from asthma, whereas 300 million suffer from the condition worldwide. Scientists aren't sure what's causing the disease in so many people, but they think the bacteria that lives in the human gut might have something to do with it. A sample of recent studies shows that bacteria found in farm dust can trigger an immune system response in the lungs of mice that may protect them from asthma, whereas babies who are given bacteria-killing antibiotics early in life have a higher risk of developing asthma. But until now, researchers haven't been able to show that gut bacteria really do a play a role in a child's likelihood of developing asthma.