There have only been around 200 cases worldwide.
It has affected US diplomats, spies, officials and family members, as well as a number of Canadian diplomatic staff deployed in Cuba.
Some who experienced Havana syndrome were left with dizziness and fatigue for months.
Before more was known about the illness, there were fears an adversary might have targeted diplomats with microwaves.
In 2016, the US was quick to blame the Cuban government for the illness.
A charge they strenuously defined.
After extensive research the US was unable to prove Cuban involvement.
According to media reports, one person was evacuated from Switzerland to the US for medical treatment after suffering from the syndrome.
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Another unproven theory is that the syndrome is related to some sort of mass condition brought about by a stressful situation.