Twenty measles cases have been reported in 11 states in the first month-and-a-half of 2024, according to the U.S. Centers for Disease Control and Prevention. Cases have been reported in Georgia, Virginia, Maryland, California, Arizona, Minnesota, Missouri, New Jersey, New York, Ohio and Pennsylvania. A cluster in Philadelphia has affected several children in a daycare.

The measles virus is one of the most contagious pathogens known to science, Kimberlin said.

“It’s like a heat-seeking missile,” Kimberlin said. “It will find people who are not immune to it.”

For decades, most people have been immune to measles due to vaccine coverage. When 95 percent of the population is immune, the virus will not spread, Kimberlin said. If coverage falls below that level, the virus can transmit among those without immunity, with devastating consequences for those who are very young and people with underlying conditions.

Symptoms can include very high fevers, up to 104 or 105 degrees. It also causes coughing, congestion and red eyes. A rash spreads, starting on the face and moving onto the torso and limbs.

Some patients experience ear infections, pneumonia and brain inflammation. That can cause seizures, hearing loss and permanent brain damage, Kimberlin said.

Kimberlin encouraged parents to catch up on vaccinations their children may have missed in the last couple of years. The measles vaccine is safe and effective, he said. Getting the vaccine can also protect others who are unable to get it because they have cancer or another illness that makes them ineligible for the shots.