Measles is one of the most contagious diseases on the planet, and it's spreading in the U.S.

Measles is one of the most contagious diseases in the world. 
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With its spiking fevers, extreme fatigue, watery eyes, diarrhea, photo-sensitivity, and possible lifelong complications or death paired with its high contagion rate, measles has been likened to the "flu on steroids." Alla Katsnelson, the associate health editor for The Conversation, shares a brief on measles, reviews its renewed spread in the U.S. and discusses the measles vaccination.

As one of the world's most contagious diseases, measles is "far more contagious than more familiar infectious illnesses such as flu, Covid-19 and chickenpox," Katsnelson explains. Beyond its ability to spread, the disease can cause severe lung problems, pneumonia or brain swelling, which usually require a hospital inpatient stay. 

Since early last year, the U.S. has had an increasing number of measles cases. Katsnelson writes, "The measles outbreak in South Carolina reached 876 cases on Feb. 3. That number surpasses the 2025 outbreak in Texas and hits the unfortunate milestone of being the largest outbreak in the U.S. since 2000, when the disease was declared eliminated here."

Measles has spread more easily in communities with overall vaccination rates below 93 to 95%, which is considered the threshold for herd immunity. Katsnelson adds, "A striking – though unsurprising – feature of the South Carolina outbreak is that at least 800 of the reported cases occurred in people who weren’t vaccinated."

The vaccine mimics what catching measles does without the long-term risks. The vaccine, when given according to the recommended 2-dose regimen, is 97% effective in preventing measles infection. Daniel Pastula, an infectious disease specialist at the University of Colorado, told Katsnelson, "The immunity from a vaccine is effectively the same immunity you get from having measles itself – but vastly safer than encountering the wild virus unprotected."

Why have some parents refused to have their children vaccinated against measles? Few people, including medical providers, have ever known anyone with measles or treated someone with it. Many parents who have lived in the U.S., where there has been herd immunity for decades, "have decided that vaccinating their children, which does pose some amount of risk from rare complications, is the better choice," Katsnelson writes. However, when enough families decline vaccination, herd immunity no longer protects against the disease.

Dr. Mehmet Oz, the administrator of the Centers for Medicare & Medicaid Services, recommended that Americans get the measles vaccine. He told Dana Bash of CNN, "Take the vaccine, please. We have a solution for our problem."