WASHINGTON • It is a mystery out of a John le Carre novel: For the past several months, United States diplomats in Cuba have suffered unexplainable symptoms, from hearing loss and vertigo to nausea and concussions.

Some say they are struggling to concentrate and recall even common words. Equally strange: While some victims said they felt vibrations or heard loud noises audible only in parts of a room, others experienced nothing.

So far, 21 Americans have reported symptoms, and Canadian diplomats are suffering as well. It has become so bad that the US decided last week to yank all non-essential personnel from its Havana embassy. Americans are being warned against visiting the country for their own safety until investigators can figure out what is going on.

For months, experts have struggled to explain what kind of weapon could cause such a wide variety of symptoms. Investigators on the scene have uncovered few clues. In the absence of hard proof, there are lots and lots of theories.

Here are some of the main ones:

SONIC ATTACK

The sonic attack theory is a popular one, especially because some of the diplomats are reporting hearing loss, sounds and vibrations.

Studies have shown that prolonged exposure to ultrasonic sound can result in hearing loss and human tissue damage.

It would take a device about a size of a matchbox to produce noise that could, at close range, induce feelings of anxiety or difficulty concentrating. But high frequency sound does not travel well through any kind of barrier, like a wall or even a curtain.

ELECTROMAGNETIC DEVICE

Electromagnetic waves can be easily directed, like a laser. They can also travel through walls, and could plausibly be concealed from afar. But these waves usually cause physical damage by heating body tissue. And the diplomats have not reported burning sensations.

CHEMICAL POISONING

There are several chemicals that can cause hearing damage, including mercury and lead, along with some industrial solvents.

But what about the other symptoms?

Writing in USA Today, director of medicine at the American Council on Science and Health Jamie Wells and microbiologist Alex Berezow explain that it is possible, particularly "if the diplomats share meals together, it is a distinct possibility that somebody poisoned their food".

THE DIPLOMATS JUST GOT SICK

Respiratory and ear infections can sometimes cause hearing loss. One inner-ear inflammation called labyrinthitis can lead to vertigo, hearing loss, bad balance, nausea and ringing in the ears - all symptoms experienced by the diplomats.

Of course, the victims have been tested for the obvious diseases, but maybe they are suffering some kind of new or mutated illness that doctors do not know to look for yet.

One reason to be sceptical: Though American diplomats work closely with Cuban staff at the embassy, only Americans got sick.

If the victims were suffering from a contagious disease, one would expect it to have spread more widely.

WASHINGTON POST