Former President George H.W. Bush and former First Lady Barbara Bush were admitted to Houston Methodist Hospital.

Bush’s Chief of Staff Jean Becker said doctors have a few theories about his condition but he is responding to treatment, the Houston Chronicle reported.

“He’s there. He’s fine and he’s doing really well,” Becker said.

According to Bloomberg, the former president was hospitalized for shortness of breath.

On Wednesday, the former President went into ICU for a procedure “protect and clear his airway that required sedation” according to the Associated Press.

The AP also reported that Barbara Bush had not been feeling well for a few weeks and decided to “take it out of committee.” A spokesperson described the move as “precautionary.”

President Elect Donald Trump tweeted his wishes to the former President and First Lady.

Looking forward to a speedy recovery for George and Barbara Bush, both hospitalized. Thank you for your wonderful letter!

— Donald J. Trump (@realDonaldTrump) January 18, 2017

Last week, Bush sent a letter to Trump saying he could not attend the inauguration since “my doctor says if I sit outside in January, it is likely I will be six feet under. So I guess we’re stuck in Texas.”

 In 2012, Bush was hospitalized with bronchitis on Christmas.

The former president is the oldest living president. In 2014, Bush made a tandem skydive for his 90th birthday.