Scientists have known for years that measles can alter the immune system – but the latest evidence suggests it's less of a mild tweaking, and more of a total reset.
It was late at night on 15 November 2019, on the Samoan island of Upolu – a tiny jade-green splodge in the Pacific Ocean, somewhere between Hawaii and New Zealand. Government officials were rushing to attend a meeting in the sleepy harbourside capital to discuss an urgent public health issue. By the end of the evening they had declared a state of emergency, with immediate effect.
Three months earlier, a member of the public had developed a characteristic red-brown blotchy rash after arriving on a flight from New Zealand, where there was an ongoing measles epidemic. They were swiftly diagnosed as a "suspected" case, but no further action was taken.
"It was really quite a surprise if you compare it to what we knew at that time from the textbooks of how measles virus would enter our host," says Rik de Swart, an associate professor of Viroscience at Erasmus University Medical Center in the Netherlands.
A decade later, an international team of researchers – including Swart – decided to take a closer look. They tagged measles with a green fluorescent protein, infected macaque monkeys with it – and tracked where the green viral particles ended up.
"[We saw that] it infects many cells systemically," says Swart. "So, this virus causes a viremia, which means that then there's virus in the blood – actually, white blood cells become infected and bring the virus to all the lymphoid tissues, which are your lymph nodes and your spleen, your thymus [a gland in the chest that's part of the immune system]," he says, explaining that this confirmed that measles is an infection of the immune system.
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