Havana Syndrome has confused scientists ever since it first emerged in Cuba in 2016. The mystery illness mostly affects American officials working in embassies abroad. They report symptoms like hearing buzzing and screeching noises, and often experience pulses of pain and intense pressure in their skulls.

Among other theories, people have suggested it’s a side effect from some unknown surveillance technology or the result of microwave weapons deployed by foreign enemies. Others argue it’s likely a mass psychogenic illness — so one that’s all in the mind — resulting from stress.

But some scientists think it might be physical, so there have been studies that look for signs of damage in the brains of people experiencing symptoms. Two of the most recent studies in the U.S. failed to detect any. So what or who is causing this illness?

We hear from Gordon Corera, the BBC’s Security Correspondent, who talks us through the some of the theories.

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