Quoting:

"Over the past several months, numerous U.S. Embassy Havana employees have been targeted in specific attacks. These employees have suffered significant injuries as a consequence of these attacks.... Because our personnel's safety is at risk, and we are unable to identify the source of the attacks, we believe U.S. citizens may also be at risk and warn them not to travel to Cuba."

Independent speculations on the cause of the injuries to State personnel range from some kind of sonic weapon (whose?) to mass hysteria.

Cuban authorities say they're investigating.

From the perspective of Cuban tourism industry, it would behoove authorities to investigate fast - Caribbean travel season is fast approaching.