In August 2017, reports began surfacing that American and Canadian diplomatic personnel in Cuba had endured unusual health-related incidents, dating back to late 2016. Twenty-two employees of the State Department reported experiencing what were referred to as "health attacks".
The Cuban government offered to cooperate with the US in an investigation of the incidents. It employed about 2000 scientists and law enforcement officers interviewing 300 neighbors of diplomats, examining two hotels and medically examining non-diplomats. NBC reported that Cuban officials stated that they "also analyzed air and soil samples, looked into whether insects could be the culprit and considered a range of toxic chemicals and the possibility of electromagnetic waves", finding nothing. The American government did not cooperate with the investigation. Cuban citizens have expressed skepticism on allegations that the Cuban government orchestrated the attacks. The Cuban Foreign Minister subsequently accused the US of lying about the incident, saying "There is no evidence, there is no evidence whatsoever, of the occurrence of the alleged incidents or the cause or origin of these ailments reported by US diplomats," adding, "Neither is there any evidence suggesting that these health problems have been caused by an attack of any sort during their stay in Cuba."
President Trump said that "I do believe Cuba's responsible. I do believe that", going on to say "And it's a very unusual attack, as you know. But I do believe Cuba is responsible."
On January 6, 2018, U.S. Senator Jeff Flake stated that the U.S. has no evidence that the illnesses experienced by U.S. diplomats were the result of attacks.
https://en.wikipedia.org/wiki/Health-related_incidents_at_the_United_States_Embassy_in_Havana