CHICAGO (NewsNation Now) — For the third time this month, a U.S. government official is experiencing symptoms of Havana syndrome.

The CIA recently evacuated an intelligence officer stationed in Serbia after suffering serious injuries consistent with the mysterious disease, the Wall Street Journal reported.

But what is Havana Syndrome?

The symptoms are often referred to as Havana syndrome because of a well-known series of cases affecting personnel at the U.S. Embassy in Cuba beginning in 2016. 26 diplomats and their families began experiencing dizziness, headache, fatigue, nausea, anxiety, memory loss and even brain damage.

Defense and intelligence agencies have ramped up investigations of what appears to be a rising number of incidents in which personnel have suffered symptoms consistent with being exposed to directed energy. The U.S. has not publicly linked the incidents to an adversary.

There are at least 200 cases under investigation, half of them involving intelligence personnel.

Recently, two possible cases of Havana syndrome delayed Vice President Kamala Harris’ trip from Singapore to Vietnam. U.S. officials said then that it was not someone who worked for the vice president or the White House.

Tracy Walder, a former CIA officer and FBI special agent, weighed in on Havana Syndrome with NewsNation’s Adrienne Bankert on Morning in America.

“I think the problem is we don’t necessarily understand exactly where Havana syndrome is coming from,” Walder said.

Watch the full interview above.