Otto Warmbier was flown home to Cincinnati on Tuesday, after spending almost 18 months imprisoned in North Korea, more than a year of it in a coma that North Korea says was caused by botulism.

According to an official North Korean report, the University of Virginia student contracted foodborne botulism shortly after his March 2016 court appearance. The report said he fell into a coma after taking a sleeping pill and had not woken up by the time he was medically evacuated from North Korea.

So what exactly is botulism?

“Botulism is caused by a bacteria known as clostridium. This bacteria produces a toxin known as botulinum toxin. This toxin can be ingested by eating improperly canned foods, which allow the toxin to grow,” Dr. Sara Siavoshi, a California neuroscientist. “It acts as toxin to the nervous system where it paralyses muscles.”

Common symptoms include a very flaccid paralysis of the arms and/or legs, difficulty breathing because of respiratory muscle weakness and blurred vision. In severe cases, it can cause respiratory failure and even cause cardiac issues. Botulism is classified as “rare” by the World Health Organization and fatal in around 3 percent to 5 percent of cases.

According to Siavoshi, there are antitoxins available to be administered to individuals who have developed botulism.

“A mainstay of therapy, however, is observing the patient for any breathing difficulties to avoid respiratory failure,” she explained. “If this does occur, they would be treated with a mechanical ventilator to help them breathe until they regain strength of their respiratory muscles.”

While Warmbier is said to have contracted his condition through food, botulism can also be contracted through wounds.

“IV drug users and cocaine users are at risk of this. Infants can contract it by being fed raw honey. Lastly, bioterrorism by spreading the toxin in the air is a rare but known cause.” Siavoshi noted.

Botulinum is also the paralyzing nerve toxin used in highly diluted amounts for the popular cosmetic anti-wrinkle solution known as Botox. It can also be used to treat heavy sweating and migraines.

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However, several officials and experts have thrown skepticism on botulism, topped with a sleeping pill, as the cause of Warmbier’s coma.

“We know that if a patient does develop signs or symptoms of botulism, they usually do not become comatose from the toxin alone,” Siavoshi said. “The toxin tends to affect the muscles and peripheral nervous system, not so much the brain. However, if one does contract botulism and already has difficulty breathing and is then administered a sedative agent on top of this, they may develop frank respiratory failure and fall into a coma due to oxygen no longer perfusing the brain.”

Last year, Warmbier was sentenced to 15 years hard labor after allegedly attempting to steal a propaganda poster from a hotel in the country’s capital, Pyongyang. An American delegation, led by a State Department representative, flew to North Korea over the weekend and insisted he be released on humanitarian grounds amid ailing health. His current state remains unknown.

“If he has actually been in a coma for months, a full neurologic evaluation would have to be made to assess the degree of brain damage. An MRI of the brain can tell how much structural damage there is,” Siavoshi added. “If he exhibits some degree of intact cognition, an extensive amount of physical and perhaps speech therapy will aid in recovery.”