Measles is highly contagious and can spread from one person to nine out of 10 people close to them, according to the CDC, which notes a person infected with measles can spread it to others four days before through four days after the rash appears. Measles can spread through coughing, sneezing and infected surfaces.

Arizona hasn't had a reported measles case so far this year, but red flags are leading some experts to worry we could see an outbreak like the one in Texas. Why it matters: Some Arizona counties have lower measles vaccination rates than in west Texas,