Measles is spreading in West Texas and New Mexico. Parents in the Rio Grande Valley want to know one thing. How bad can this get.

Here is the short answer. Measles is one of the most contagious sicknesses on Earth. It spreads faster than Ebola. It spreads faster than smallpox. It beats almost every other disease doctors track.

Before the measles shot came out, almost every kid in America caught measles. The disease killed 400 to 500 children a year in this country. The shot changed that. But the shot only works if enough people get it. In parts of Texas, fewer people get it every year.

Scientists use a tool to measure how fast a sickness spreads. They call it the basic reproduction number. Most people just say R naught.

R naught means one simple thing. It is how many people one sick person can pass the germ to. That is if nobody near them has any protection.

For measles, that number is huge. One sick kid can infect 12 to 18 other kids. Picture a full kindergarten class. One sick kid could reach almost every other kid in that room.

Now compare that to Ebola. One sick Ebola patient infects about 2 people. Measles spreads six to nine times faster than that.

Justin Lessler studies how sickness spreads. He works at the University of North Carolina Gillings School of Global Public Health. He said a different number matters more in real life. It is called the effective reproduction number. That number looks at how many people a sick person can infect right now. It counts real vaccine rates in a real town.

"Are we at risk of getting, like in the old days, thousands or tens of thousands of cases from this outbreak?" Lessler said. "That's probably pretty unlikely. The vaccination firewall is still pretty strong at that sort of broad level."

That is the good news. Here is the part that should worry South Texas parents.

In Gaines County, where this outbreak started, only about 82 out of 100 kindergartners got the measles shot. Doctors say a town needs 95 out of 100 people vaccinated. That stops measles from spreading. Gaines County sits far below that mark.

This is not just a West Texas problem. Across the whole country, measles vaccine rates dropped to 92.7 percent. That number comes from the Centers for Disease Control and Prevention. It sits below the safe mark too. About half of all Texas counties now fall under 95 percent. Some of those counties sit close to home.

When shot rates drop, the wall of protection gets weaker. Fewer vaccinated kids means more open paths. The virus can jump from one classroom to the next.

Matt Ferrari teaches biology at Penn State University. He runs the school's Center for Infectious Disease Dynamics. He compared this outbreak to COVID-19.

The first strain of COVID had a reproduction number of about 3. That strain still spread across the whole world.

"It's not unreasonable to think that measles could spread as fast as pre-vaccine SARS-CoV-2 in populations, particularly in school settings, where kids are vaccinated at 80-85%," Ferrari said.

That number, 80 to 85 percent, sits close to what Gaines County has right now.

Lessler gave a warning too. He said if shot rates keep dropping, big outbreaks could return. Thousands of cases a year could come back within five to ten years. For now, doctors say the vaccine wall still holds. But they say it holds a little less each year.

For South Texas families with kids in school, the math stays simple. More vaccinated kids in a classroom means a safer room for every kid. That includes kids who cannot get the shot for medical reasons.