WASHINGTON - American diplomats and their spouses residing in Cuba began reporting symptoms that included hearing loss, dizziness, balance and visual problems, headaches and cognitive issues last December.
By late January, the US State Department realised that the illnesses were related.
The number of personnel thought to have been affected has gradually grown over the months, and on Tuesday the State Department added one more to the list, bringing it to 22.
Canadian diplomats are suffering as well.
Some of those who were affected reported hearing odd sounds in particular rooms of their homes, leading some experts to speculate that some kind of sonic weapon or faulty surveillance device may have been at fault.
One possibility being explored is whether the diplomats were made ill by a "sonic attack," though the State Department has refrained from using that term.
What is a sonic weapon or attack?
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.
How would it work?
The frequency of the sound could be high - ultrasound - or low - infrasound.
Ultrasound frequencies above 20,000 Hz, or 20kHz, are inaudible to humans but can damage parts of the ear, says the BBC.
However, Steve Goodman, author of the book Sonic Warfare, told BBC Radio 4 that it was not clear whether inaudible soundwaves could give someone the hearing loss the state department described.
"The information given is so vague it's hard to say," he said.
Infrasound or lower frequencies below 20Hz are used by animals to communicate. If very loud, it can lead to vertigo or cause vomiting. It could also lead to uncontrollable bowel movement if very intense.
However, an attack using infrasound would need to use a large array of subwoofers and therefore wouldn't be very covert, according to the New Scientist.
What damage can a sonic attack cause?
Dizziness, hearing loss, headaches, vomiting, vertigo and brain damage could be some of the symptoms of an acoustic attack.
Who could be behind the attack?
FBI agents investigating in Cuba have not been able to find the causes of the ailments. With little hard evidence in hand, former US officials who follow Cuba closely have entertained a handful of theories that involve a third country, such as Russia - which would have an interest in driving a wedge between Cuba and the United States as the election approaches - or North Korea, which has an embassy in Havana.
Neither scenario seems plausible without some level of Cuban government complicity, these officials say, considering how rigorously Cuba's intelligence service tracks diplomats on the island.
Cuba has denied involvement in any attacks and says it has reinforced security for US diplomatic personnel.
Has sound been used as a weapon earlier?
Yes, sound has been used as a nonlethal, yet potentially harmful, weapon, says CNN.
In several countries, including Britain, it has been used to prevent young people from loitering. Referred to as the mosquito, it produces a very high-pitched sound that is audible to teenagers, but adults cannot percieve it. Hearing it can lead to feelings of discomfort and nausea.
"It's not audible to adults because your hearing fades as you get older," James Parker, a sound and law expert at the University of Melbourne, was quoted as saying by CNN.
Long-range acoustic devices, or LRADs, have also been used by authorities to control crowds of people. Most recently the devices, which emit a loud, painful sound over a long distance, were used in 2014 to disperse protestors in Ferguson, Missouri after the fatal shooting of Michael Brown.
The US army has used heavy metal and children's music to make Iraqi prisoners of war co-operate in interrogations, and deprive them of sleep, the BBC reported.
SOURCE: NYTIMES, WASHINGTON POST, BBC, REUTERS, CNN