Using these little bumps, called fusion proteins, measles enters white blood cells and makes copies of itself. As the white blood cells travel throughout the body, they inadvertently spread the viral particles to organs such as the lungs, liver and, in rare cases, the brain.

Measles is not often seen in the United States, but Americans are growing more concerned about the preventable virus as cases continue to rise in rural West Texas.

Vaccination eliminated measles from the U.S. 25 years ago. But it can still spread in pockets where vaccination rates are low, like the west Texas county with a current outbreak. Here's how fast.

The growing measles outbreak in the U.S. Southwest claimed a second life this week. So far this year, more than 220 cases have been reported across 12 states, with the majority in Texas. John Yang speaks with infectious disease epidemiologist Jessica Malaty Rivera to learn more about the outbreak and how it differs from previous ones in recent years.