The 28-year-old has not taken part in either of Midtjylland's most recent fixtures since the Danish Superliga winter break, most recent appearing in a 5-1 win over Viborg on December 4.
Midtjylland have taken on Brondby and Aarhus this month without the services of Olsson, but the club did not specify a reason for his absence until now, revealing that he collapsed at his home on February 20, two days after the 1-0 loss to Brondby.
In a statement, Midtjylland revealed that Olsson is currently on ventilation at Aarhus University Hospital, and the club felt "compelled" to make an announcement as rumours about his absence began to swirl.
"Against the background of the increasing rumors and conjectures that are pending about the reason for Kristoffer Olsson's absence in the recent period, we feel compelled to go out with this announcement to the public," Midtjylland said.
"The 28-year-old Swedish national team player lost consciousness at his home on Tuesday 20 February and was transferred to Aarhus University Hospital, where he is hospitalized and on a ventilator.
"Kristoffer Olsson is affected by an apparently acute disease related to the brain, which is not due to self-harm of any kind, nor is the cause due to external factors. A team of Denmark's leading medical experts is currently working at high pressure to make a diagnosis and initiate the right treatment.
"Since admission, Kristoffer has been surrounded by his immediate family and parts of FC Midtjylland's staff, and is being treated by specialists in the field. FC Midtjylland urgently requests the public to show respect and understanding, so that Kristoffer, the family and the doctors get the necessary peace of mind to ensure Kristoffer's recovery and recovery as well as possible.
"Everyone at FC Midtjylland is of course deeply affected by Kristoffer's sudden illness and our thoughts and full support go out to Kristoffer and his family. FC Midtjylland has no further statements at present, but we will inform the public when significant new information arises."
Olsson's former team Arsenal sent their well-wishes to the Sweden international on social media following news of his hospitalisation, writing: "Get well soon, Kristoffer. We wish you a full and speedy recovery."
Sweden also offered a message of support on X - formerly known as Twitter - saying: "We're with you, Koffe. On Tuesday morning, we received the terrible news that the men's national team player Kristoffer Olsson suffered a sudden illness and is now on a ventilator in hospital. The entire Swedish football family is thinking of you and hopes that you will recover soon. Our thoughts also go to Kristoffer's loved ones at this difficult time."
Olsson came through the Norrkoping academy in his homeland before transferring to Arsenal in 2011, and he made one senior appearance for the Gunners in a 2013-14 EFL Cup win over West Bromwich Albion, scoring from the spot as Arsenal beat the Baggies on penalties.
The midfielder left Arsenal permanently in 2015 to join Midtjylland, since when he has also represented AIK Fotboll, FC Krasnodar and Anderlecht before making a permanent return to Denmark last summer.
Olsson set up five goals in 20 games in the first half of the 2023-24 season and has won 47 caps for the Sweden national team, starting all four of his nation's matches at Euro 2020.