The mysterious ailment known as “Havana syndrome” began afflicting U.S. diplomats and intelligence officers in Cuba’s capital, Havana, in late 2016. Victims reported the sudden onset of a range of symptoms such as headaches, nausea, memory loss and other cognitive difficulties. The initial cluster of cases confounded medical experts.

More than five years later, as many as 200 incidents have been reported among U.S. personnel in a list of countries that includes Russia, China, Colombia, Uzbekistan and even the United States itself.

However, the Central Intelligence Agency in January cast doubt upon the widespread belief that a foreign adversary was behind the syndrome. The agency called such a scenario “unlikely” and suggested that all but a “few dozen” cases could be attributed to preexisting medical conditions or environmental or other factors

“We assess it is unlikely that a foreign actor, including Russia, is conducting a sustained, worldwide campaign harming U.S. personnel with a weapon or mechanism,” a senior CIA official, speaking on the condition of anonymity under ground rules set by the agency, told The Washington Post.

Here’s what we know about Havana syndrome.