1. Measles What it is: A highly contagious viral infection that involves the respiratory system, including the lungs and breathing tubes. How you get it: The measles virus gets into the air when someone who has it coughs or sneezes. It can also last for up to 2 hours on something they touched. Most people who aren't immune -- 90% -- will get it if they are near an infected person. Why it’s serious: Measles can cause pneumonia, brain swelling, and death. Before the vaccine, 3 million to 4 million people in the U.S. got measles each year, 48,000 were hospitalized, and 400-500 died.