The mysterious illness now 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.

Now, nearly 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.

The president of Colombia said he is aware of reports of Havana Syndrome cases at the U.S. Embassy in Bogotá, affecting at least five families with links to U.S. staff in the last few weeks. President Iván Duque said Tuesday that the investigation will be left in the hands of U.S. authorities ahead of a trip scheduled to the country by Secretary of State Antony J. Blinken.

Last month, a member of CIA Director Bill Burns’s team reported experiencing symptoms consistent with Havana Syndrome while traveling to India. In August, two U.S. personnel in the Vietnamese capital, Hanoi, reportedly suffered from “unexplained health incidents” just before the arrival of Vice President Harris. Germany has also recently confirmed an investigation into an “alleged sonic attack” against U.S. Embassy staff in Berlin.

Here’s what we know about Havana Syndrome, its potential causes and who may be behind it.