Measles continues to spread in West Texas and New Mexico, with more than 250 people infected — many unvaccinated school-age children. Two cases in Oklahoma, for which state officials have not provided a location, have also been linked to these outbreaks. Eleven other states have reported isolated measles cases, typically linked to international travel.

The Texas outbreak began in late January, when local health officials reported two cases in Gaines County, a rural agricultural area on the western edge of the state.

The outbreak quickly escalated. Measles has spread into neighboring counties and infected at least 223 people as of Tuesday, including 29 people who have been hospitalized and an unvaccinated young child who died, the first such death in the United States in a decade.

New Mexico also declared an outbreak in Lea County, which borders Gaines County. While the cases in New Mexico have not officially been connected to the Texas outbreak, officials have said they are “undoubtedly related.”

Last week, an unvaccinated resident of Lea County who tested positive for the virus died, though officials have not yet confirmed that measles was the cause of death.

In both states, most measles cases have spread among people who are unvaccinated or with unknown vaccine status.

For years, Gaines County has had low childhood vaccination rates, largely because of the area’s large Mennonite community. While there is no religious doctrine that explicitly forbids vaccines, the insular Christian group has historically avoided interacting with the health care system and has a long tradition of using home remedies and supplements.

Roughly 82 percent of the kindergarten students in the county had received the measles-mumps-rubella vaccination, or M.M.R., last year, which is far below the 95 percent coverage needed to prevent outbreaks in a community.

Texas public schools require children to have received certain vaccines, including the M.M.R. shot, but parents can apply for an exemption for “reasons of conscience,” including religious beliefs. Last year, Gaines County had one of the highest exemption rates in the state.

Vaccination rates can vary widely by school district. The Loop Independent School District in Gaines County is a small district with one school and the lowest rate of measles vaccination in the affected Texas counties. Only 46 percent of kindergarten students had received their M.M.R. vaccine in the 2023 school year, down from 82 percent in 2019.

In Lea County, N.M., the M.M.R. vaccination rate for children and teens is relatively high, at about 94 percent. But the rate among adults is much lower: 63 percent have received one shot of M.M.R., and only 55 percent have received both shots, according to local health officials, though they noted that there may be vaccinated adults whose records have not been added to the system. Adults make up more than half of reported cases in New Mexico.

Measles is one of the most contagious known infections. In a hypothetical community where nobody had immunity from the virus and each infected person infected 18 others, a small outbreak would quickly grow out of control:

Every vaccinated person would help slow the spread of the virus and limit the outbreak:

For the outbreak to end quickly, each infected person would need to infect fewer than one other person, on average. In this example, 17 of 18 people would need to be vaccinated — more than 94 percent of the community:

And while measles symptoms often resolve on their own within a few weeks, the virus can be extremely dangerous in rare cases. It may cause pneumonia, making it difficult for children to get oxygen into their lungs.

The infection can also lead to brain swelling, which can cause lasting damage, including blindness, deafness and intellectual disabilities. For every 1,000 children who get measles, one or two will die, according to the Centers for Disease Control and Prevention.

The virus also causes “immune amnesia,” making the body unable to defend itself against illnesses it has already been exposed to and leaving patients more susceptible to future infections.

Once someone is infected with the virus, doctors have little control over how serious a measles infection becomes — there is no antiviral for measles to stop its spread in the body, only “supportive care” to help manage symptoms. That’s why experts recommend that people get two doses of the M.M.R. vaccine, which are 97 percent effective at preventing infection.

Measles was officially declared eliminated — which means the virus is not continually spreading — in 2000, in large part because of aggressive vaccination campaigns. Experts now fear that status may be at risk, as childhood vaccination rates have been falling nationally.