Eman, our informally adopted son who is a physician in Monrovia, Liberia, is recovering from the measles. He contracted it from a teen patient.
“It started with a slight fever and chills, and became a burning fever. Then I had cough and red, watery eyes. I treated myself for malaria and went for a checkup. The doctor saw the rashes on my face and hands, and said it didn’t look like a typical measles rash. Initially I was treated as an outpatient and told to isolate. Then in two days, I got shortness of breath at home, and had to get admitted. I experienced respiratory distress and had to get supplemental oxygen. It reminded me of having COVID.”
He had measles vaccine as a child, and thought he had some cross-protection from having had chickenpox.
“I’ve been looking up papers on why a previously vaccinated person or an adult would get measles. A lot of them are pointing to incomplete dosages, improper handling of the vaccines, waning immunity, or being immunocompromised. There are many other previously vaccinated adults with measles for the first time. This should be happening in mostly immunocompromised people, and I’m not. The measles outbreak has been terrible.”
Perhaps the scariest thing about infectious diseases is that they change. That’s what nucleic acids – DNA and RNA – do.
Measles in Many Nations
Liberia has faced measles outbreaks before, a particularly serious and widespread one in 2022. And the disease is creeping back in many places.
Health officials in Australia fear loss of the nation’s measles-free status, with recent cases attributed to people returning from visiting southeast Asia, where they became infected.
The disease is on the rise in Indonesia, Thailand, and Vietnam, and is endemic in India, Bangladesh, Myanmar, Nepal, Angola, Yemen, and Mexico. Cases have recently been reported from Bolivia, Canada, Guatemala, and Mexico.
“Measles elimination in the Americas was a historic achievement, but recent events demonstrate that this progress is fragile and can be reversed without consistently high vaccination coverage. Coordinated action now can save lives, prevent further outbreaks, and ensure the health security of the region,” said Jarbas Barbosa, director of the Pan American Health Organization, in a report issued February 3.
And of course, measles is returning to the US.
The CDC reported 2,285 confirmed measles cases in 2025, and as of March 26, 2026, 1,575 cases, from 31 states. The new year has seen 16 new outbreaks.
The Natural Cycle of Mutation and Selection
Two interpretations of Eman’s experience – contracting measles despite having been vaccinated – are possible.
An anti-vaxxer might say “Measles vaccine doesn’t work.”
A biologist like me would say “Pathogen genomes mutate.”
A viral genome changes, as all genomes do, as the genetic material replicates and occasionally, errors happen. That’s mutation, a natural process arising from the underlying chemical reactions. If new viral variants arise that can evade vaccine protection, they may come to persist. Then cases of the infectious disease start popping up. That’s selection.
The cycle of mutation and selection is about as basic as biology gets.
But measles is especially dangerous, for it can shut down immunity to past, non-measles infections, too. This recent DNA Science post explains the details.
The Fear Factor
Perhaps I appreciate vaccines because I remember having the diseases that some of them now protect against.
Getting the handful of “childhood diseases” was just part of being a kid in the 1960s. I had measles for a month, German measles (aka rubella), mumps, and chickenpox. I recounted how my younger sister and I straddled the time of the invention of the measles vaccine here – I had the disease, she had the shots. Now that measles is back, I’m likely still protected – she may need an MMR booster.
Why are infectious diseases once almost completely gone returning?
Related on the SLP
Because people are actually encouraging it, their fear overwhelming any knowledge of biology. And they may listen to non-experts with impressive titles and in positions of power (See A Pop Quiz in Biology for RFK J).
Individuals choosing not to vaccinate their children are enabling nature to take its course, providing a breeding ground for the measles virus to spread and change, shattering herd immunity. And as a result, resistance mutations, which arise naturally, are more likely to persist, perhaps to flourish. Will the virus mutate enough for even natural immune protection to become insufficient?
Ignorance and selfishness are a potentially deadly combination.
Ricki Lewis is a science writer with a PhD in genetics. She regularly contributes articles to PLOS Blogs: DNA Science. She is author of the textbook Human Genetics: Concepts and Applications and The Forever Fix: Gene Therapy and the Boy. Follow her on her website
A version of this article was originally posted at PLOS Blogs and has been reposted here with permission. Any reposting should credit the original author and provide links to both the GLP and the original article. Find PLOS Blogs on X @PLOS
