Americans don’t think about measles because vaccination had largely eliminated the scourge from the U.S. in 2000. Many doctors cannot even diagnose measles because they have not seen it in practice. But measles outbreaks are back. There have been more cases this year than in each of the past two years. The measles virus is one of the most contagious on Earth; it can live for up to two hours in the air after an infected person coughs or sneezes. Up to 9 out of 10 people who are unvaccinated will become infected if they breathe the contaminated air or touch a surface that has been infected.