In 2000, measles was declared eliminated in the United States. Twenty-six years later, we are talking about it again.

I have only ever known this disease as a box checked off on my childhood vaccination records, so its return feels almost out of place.

This year, thousands of cases have been reported across the country, with outbreaks reaching communities that haven’t seen widespread transmission in decades.

How does a disease come back after we supposedly got rid of it?

The answer has less to do with the actual measles virus changing and more to do with the conditions surrounding it. Measles never disappeared from the world, and because it is one of the most contagious viruses we know of, keeping it under control depends on maintaining an unusually high level of immunity within a community. Gaps in that protection can grow, ultimately allowing the virus to return.

To understand this, it is important to know what makes measles so remarkably good at spreading.

Measles is a respiratory virus that spreads through the air when an infected person coughs, sneezes, talks or even breathes. Virus-containing particles can remain in the air for up to two hours after an infected person has left the area. Among people without immunity, roughly 90 percent of those who are exposed will become infected.

Epidemiologists measure this contagiousness using a number called R0. It estimates how many people one infected person can transmit a disease to in a population with no immunity. Measles has an estimated R0 between 12 and 18, meaning a single infected person could theoretically spread the virus to 12 to 18 others. For comparison, estimates for the original strain of COVID-19 were closer to 2 to 3.

This extraordinary ability to spread is why even relatively small gaps in immunity can make a difference.

The measles, mumps and rubella vaccine, better known as the MMR vaccine, is highly effective. Two doses are about 97 percent effective at preventing measles. But vaccination also affects more than the individual receiving it. When enough people in a community are immune, the virus has fewer opportunities to find its next susceptible host. Eventually, transmission chains are interrupted.

This is the basic idea behind herd, or community, immunity.

Because measles spreads so easily, approximately 95 percent vaccination coverage is generally needed to maintain strong community protection. But nationwide MMR coverage among U.S. kindergartners has fallen below that level in recent years.

A difference of only a few percentage points may not seem significant, but across millions of children, it can leave hundreds of thousands susceptible.

Vaccination rates can vary between states, neighborhoods and even individual schools. A community may exist within an otherwise highly vaccinated area where a much larger percentage of people are susceptible. If measles enters one of these pockets, it can move quickly.
This also helps explain what scientists actually mean when they say measles was “eliminated.” Elimination does not mean eradication. Measles never disappeared worldwide. Elimination meant that continuous transmission of the virus within the United States had been stopped for at least 12 months.

Cases can therefore still enter the country through international travel. When an imported case reaches a highly immunized community, the virus struggles to spread very far. But when it reaches a population with gaps in immunity, a single case can become an outbreak.

The infection commonly begins with a high fever, cough, runny nose and red or watery eyes before the characteristic rash appears. In some cases, measles can lead to pneumonia, severe dehydration and encephalitis, or inflammation of the brain. Young children and people with weakened immune systems are particularly vulnerable to complications.

Perhaps one of the most fascinating consequences of measles is something researchers call “immune amnesia.” Our immune system normally maintains memory cells that recognize pathogens we have encountered before. Measles can destroy portions of this immune memory, essentially making the immune system “forget” some of the infections it previously learned how to fight. Even after someone recovers from measles itself, they may temporarily be more vulnerable to other infections.

There is a strange irony in successful public health efforts. The better we become at preventing a disease, the less visible its consequences become. Eventually, generations grow up without seeing what that disease once looked like, and it becomes easy to assume the threat disappeared on its own.

But measles was not eliminated because it stopped being contagious or dangerous. We eliminated it because we became good at stopping it from spreading.

Its return reminds us that, sometimes, keeping a disease in the past requires continuing the very measures that put it there.