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.