Marc Polymeropoulos woke up in his hotel room with his head spinning and ears ringing. "I felt like I was going to vomit. I couldn't stand up. I was falling over," he recalls. "I have been shot at numerous times and this was the most terrifying experience in my life."

Polymeropoulos had spent years in Iraq, Syria and Afghanistan as a senior officer of the CIA fighting America's war on terrorism. But that night in Moscow he believes he was targeted by a secret, microwave weapon.

After Russia's interference in the 2016 US presidential election, CIA leadership issued a "call to arms" and redeployed battle-hardened officers like Polymeropoulos to push back.

He would eventually become acting chief of clandestine operations in Europe and Eurasia, working with allies to expose Moscow's activity, including the 2018 poisoning of former Russian spy Sergei Skripal in Salisbury, England.

In December 2017 he visited Moscow, but not undercover. He wanted to use a regular "liaison" meeting between Russian and US spies to see the country for himself. He was not there, he insists, for any clandestine activity. The Russians had not been keen on him coming, but acquiesced.

It was early on during the trip that he fell ill. On his return to the US the vertigo went, but other symptoms persisted to this day. "I've had a migraine headache for three straight years. It has never gone away," he told the BBC. He was unable to work a full day and took months off, starting a long medical journey.

His suspicions arose because, from 2016, diplomats in Havana, Cuba, reported similar symptoms - as did some Canadians.

Sometimes it was the sudden onset of a loud noise leading to intense pain, while others felt pressure on the head leading to dizziness and vertigo. The sensations seemed to come from a particular direction in a specific location. This became known as "Havana syndrome".