Maybe they have a cold or the flu. Other people have asthma. This makes it hard for oxygen to get to the air sacs. Some people have cystic fibrosis. It causes mucus to collect in their lungs.
Scientists look for ways to fight diseases that affect our lungs.
Some scientists are studying cystic fibrosis. They have found the gene that causes the disease. Other scientists are studying asthma. They want to help children avoid asthma attacks. Saliva is a watery, colorless which is produced by some glands inside our mouth.