It is one of the greatest mysteries in all of sports. Lionel Messi, arguably the world’s greatest soccer player, frequently vomits on the pitch while competing in matches. It is not some new phenomenon, nor does it only happen in big games, it happens in training, friendlies, even in the confines of Messi’s own home.

“I don’t know what it is. But I had a thousand exams,” Messi has said. “I start to feel nauseous to the point where I almost vomit, and then it goes away.”

Messi’s regurgitation was perhaps most famously documented in 2011 during the Spanish Super Cup final between Barcelona and Real Madrid. With a major trophy on the line, Messi began to vomit on the pitch, late in the second half of the 2-2 game. With less than ten minutes to play in regulation, Barcelona fans and medical staff alike were concerned when Messi became sick. He stayed on the pitch however, and minutes later scored the game-winning goal in the 88th minute to seal the Super Cup trophy for his team.

Messi would continue this bizarre behavior throughout the years, in matches for both his club team, Barcelona, and the Argentina national team. Against Germany in the first half of the World Cup Final in July, Messi was seen dry heaving and vomiting liquids on the pitch. The vomiting does not interfere with Messi’s play on the pitch, nor does it worry his teammates or managers.

“We’re not worried at all,” said Barcelona boss, Luis Enrique. “Either medically or in football terms. I don’t give these events the slightest importance.”

For years however, the reason Messi would vomit has been a mystery to fans, players, coaches and the medical community alike. There have been speculations such as acid reflux disease, diet, bulimia, and nerves.

“Nerves,” guessed his Argentina National team coach, Alejandro Sabella, said during the 2014 World Cup in Brazil. “I reckon that in these moments there is anxiety more than anything.”

Rampant speculation aside, the mystery has remain unsolved…until now. According to the Spanish Television program, La Goleada, Messi’s vomiting at random times on the pitch is due to a chronic medical condition known as “Chronic sinusitis,” or “chronic rhinosinusitis”. According to the encyclopedia of diseases and other medical conditions, it is a common, non-threatening condition that affects the cavities around the nasal passages or sinuses. When the patient is going through chronic rhinosinusitis, the sinuses are inflamed and swollen, therefore causing mucus to build up.

As the mucus build up, it becomes difficult to breathe and thus the dry heaving and vomiting can occur, especially while doing an athletic activity in order to try and clear the sinuses. The condition can last up to 8 weeks when it occurs.

“Mucus production makes Messi start to liquefy so he starts spitting and blowing to try and get it out. After minutes have passed, and his body is unable to remove the mucus, that’s when he starts to dry heave. After two or three attempts, vomiting occurs,” said Dr. Helena King.