Last week, another Canadian diplomat was diagnosed with a mysterious disease so weird it's been referred to in some circles as "the thing."

The illness afflicts only government employees from the U.S. and Canada. Sufferers report feeling pulsing or hearing a ringing in their ears. Then headaches, dizziness, trouble concentrating and struggles to remember basic words.

Diplomats have been complaining about "Havana syndrome" for two years. But, though the U.S. has sent CIA and FBI officials to investigate, we know very little about what or who is behind it.

The newest case marks the 13th time a Canadian officer or family member has reported these "unusual health symptoms." More than 20 Americans have also been affected. The Canadian government has said it will allow all staff posted in the Cuban Embassy to return home "if they wish."

When was it first reported?

On Dec. 30, 2016, a CIA agent operating in Cuba stopped by the U.S. Embassy's health office. According to the New Yorker, the patient described "strange sensations of sound and pressure while in his home, followed by painful headaches and dizziness." About a week later, the spy complained of another attack.

What happened next?

U.S. officials told the New Yorker they weren't sure how seriously to take things at first. "It's like serial killers," a former State Department official said. "It usually takes three or four before police conclude 'Wait a minute, these are connected.' "

Soon, a pattern emerged. By February, two more CIA officers reported the same sensations. By spring, 16 people had reported symptoms. By the fall, another five Americans were afflicted. The attacks were unusual — some reported hearing sounds; others said they felt a pulsing, followed by pain. People were afflicted at home, in hotels, in temporary residences.

Around the same time, Canadian officials began to report some symptoms. A Canadian diplomat and his wife were awakened one night by a feeling of waves of pressure. Their children had nosebleeds. Eventually, as many as 12 Canadians were afflicted. Canada's foreign service began warning staff against bringing their family on assignment. The government also began housing the diplomats together, far from their U.S. counterparts. Initially, it seemed the attacks had stopped, at least until now.

What do experts think?

Victims reported headaches, dizziness, confusion and the inability to think straight. Doctors who've conducted brain scans of the victims say it looks as if they suffered a concussion without the preceding traumatic injury.

It's unclear how the victims might be suffering these injuries. Some have posited that the assailants are using a "sonic weapon," but U.S. investigators have allegedly ruled out the possibility that the injuries were caused by the sounds. Other experts — including a senior doctor who examined the victims — have suggested microwave rays. But as the Post reported, "no microwave weapon that affects the brain is known to exist."

Others, including a specialist in neuro-weapons at Georgetown University, suggested a device that emitted radio frequencies or electromagnetic pulses that enter through the ears of the victim.

Who could be behind it?

Some suspected Cuban hard-liners loyal to Fidel Castro, acting covertly to try to disrupt the normalization of relations. But if that's what's going on, experts say, the dissidents are probably acting in conjunction with another hostile government. For one thing, the CIA has said it doesn't know what kind of weapon could cause these symptoms; the team that developed it would probably have access to sophisticated machinery.

That leaves a couple of prime suspects: China and Russia. Many say Moscow is the most likely culprit. Russia had a covert weapons program for decades, and the United States said the country once tried to harness microwave radiation for ill.