The United States’ withdrawal from the World Health Organization became official Thursday, formalizing a fissure between the Trump administration and the Geneva-based global health agency that dates back to the early days of the Covid-19 pandemic.

Thursday marks the one-year anniversary of the date on which the WHO was informed that President Trump had decreed that the U.S. would terminate its membership in the organization, something he tried to do during his first term in office. According to a joint congressional resolution passed in 1948 to allow the United States to join the WHO, the country had to give a year’s notice before withdrawing. (The joint resolution also stipulated that the country had to pay outstanding bills before leaving, a condition that has not been met.)

After a year of ongoing measles outbreaks that have sickened more than 2,400 people, the United States is poised to lose its status as a measles-free country. However, the newly appointed principal deputy director at the Centers for Disease Control and Prevention, Ralph Abraham, said he was unbothered by the prospect at a briefing for journalists this week.

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Scott Rivkees still remembers two cases from his first week as a pediatric intern at Massachusetts General Hospital in 1982.

The first was a patient with chickenpox or varicella encephalitis, a virus that causes brain inflammation. The second was a child with bacterial meningitis, a severe infection that causes inflammation of the membranes around the brain and spinal cord. The infection can produce seizures and deafness and lead to death.