Measles should be long forgotten. Why are cases rising in the U.S.?

For decades, measles was a rarely seen disease in the U.S., but now it's back on the rise, and the U.S. is at risk of losing its “elimination status,” Centers for Disease Control and Prevention officials warned in a report on Thursday. Rates of the highly contagious infection began to tick back up in 2019, but a third of the new cases identified in the past four years were detected in just the first three months of 2024, the CDC said. It's a trend that has health officials concerned, and measles isn't the only disease of the past that's rearing its head again.

U.S. tuberculosis cases reached their highest level in a decade in 2023, and this year, infections have shown up in schools in Pennsylvania and Las Vegas, and upticks have caused alarm in Chicago and California. Polio, a virus that can cause paralysis in its most severe form and was once one of the most feared diseases in the country, was even detected in New York's Rockland County in 2023 for the first time in a 10 years.

So what's behind this resurgence and where is it happening? Infectious disease experts explain.

Where are measles cases on the rise?

Measles has gotten the most attention after emerging on both coasts in a matter of weeks and affecting a total of 121 people so far this year. According to the CDC, measles cases have been reported in 17 states so far in 2024: Arizona, California, Florida, Georgia, Illinois, Indiana, Louisiana, Maryland, Michigan, Minnesota, Missouri, New Jersey, New York, Ohio, Pennsylvania, Virginia and Washington.

Many cases are linked to places where people who may not be vaccinated are in close contact with one another, or to international travel. Chicago has had 61 cases in 2024 since the virus began spreading at a migrant shelter. Georgia confirmed its third case of 2024 on Thursday after an unvaccinated person who'd recently traveled internationally tested positive. Visits to Florida resulted in cases in at least three additional states — Indiana, Louisiana and Ohio — according to documents obtained by CBS News in late March. In January, public health officials in Virginia warned about possible measles exposure after a person with the disease traveled through Dulles International Airport on Jan. 3 and Ronald Reagan National Airport on Jan. 4.

Schools, day cares and family gatherings are also vulnerable to the spread of measles. Nine people in the Philadelphia area tested positive for measles after an outbreak started at a day care in late December 2023. On Jan. 10, health officials in Washington state urged health care providers to be on the lookout for measles symptoms after six people contracted the illness at a family gathering.

The CDC also issued a warning to health care providers about measles cases rising in the U.S. and globally and cautioned labs to take care when testing and analyzing samples from people with symptoms.

What's going on here?

Not surprisingly, a lot of it has to do with not getting vaccinated, Dr. William Schaffner, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, tells Yahoo Life. "There are some parents who have withheld their children from vaccination," he says. "It's vaccines that have eliminated these diseases. If you withhold children from vaccination, you will create pockets of susceptibility." (Worth noting: The U.S. does not currently vaccinate against tuberculosis, given the low risk of exposure.)

About 95% of people need to be vaccinated against measles to prevent it from reemerging. In 2022 (the latest data available), 83% of people had received a dose of measles vaccine, falling from 86% in 2019, the CDC said.

"As a whole, vaccination rates in this country have drifted downward," Dr. Thomas Russo, professor and expert in infectious diseases at the University of Buffalo in N.Y., tells Yahoo Life. "In part, it's due to the anti-vaccination movement, but some people missed doses during the pandemic."

People are also traveling again post-pandemic, including to areas of the world where measles and other diseases are more common than they are in the U.S., bringing them back when they return, Russo says.

Many have forgotten what these diseases are like and don't feel a sense of urgency in protecting themselves or their families against them, Schaffner points out. "When these diseases are reintroduced to our population, there is always a great surprise at how readily they spread and how ill they can make people," he says. "This historical information hasn't been passed down because we haven't had these diseases in years."

How dangerous are these outbreaks?

Dr. William A. Petri, an infectious disease expert at UVA Health, tells Yahoo Life that cases of these diseases still remain low in the U.S. "Measles remains unusual in the U.S. — less than 100 cases per year on an average year," he says. Petri also points out that there was a "modest" increase in tuberculosis cases in the U.S. in 2022.

But people who get these diseases can get really sick. "Current parents of young children don't have a clear understanding of how serious these diseases are and they trivialize them," Schaffner says.

Measles is considered one of the most contagious diseases, Russo notes, making it easy to spread quickly among unvaccinated people. "Some of these diseases are potentially lethal," he says. "Polio is much less common than measles, but it can have significant consequences, such as paralysis." Tuberculosis can cause a cough that lasts longer than three weeks, coughing up blood and fever, and can be fatal if it's not treated properly, according to the CDC.

"These are childhood diseases we kept at bay, and they're now making a comeback," Russo says.

When to get vaccinated

Tuberculosis vaccines are only recommended under select circumstances in the U.S. But vaccines against polio and measles are part of routine childhood vaccinations, Petri says.

If you're an adult who hasn't been vaccinated against these diseases, doctors recommend changing that. "Your primary care physician or health department can give you these vaccines," Petri says. Your doctor may not keep the MMR vaccine (which targets measles, mumps and rubella) or inactivated polio vaccine (IPV) in stock, so it's worth calling in advance to see if they can be ordered for you.

"It's not too late to get these vaccines as adults," Schaffner says. Russo agrees, adding: "The consequences of these diseases can be even more severe in adults than in children."