In an official statement, the club confirmed that the manager was admitted to hospital last night and remains in "a comfortable condition".
It added that he is "currently undergoing tests" and "it is anticipated that he will remain in hospital for several days".
Assistant manager Gary McAllister will take charge for the home game with Stoke on Saturday. The club said that Houllier "wishes to convey his appreciation to Aston Villa supporters for their concern and good wishes."
Villa chief exec Paul Faulkner, who spoke with Houllier this morning, said: "On behalf of the football club, Mr. Lerner (chairman) and I wish to convey our best wishes to Gerard. I spoke with him this morning, as did Gary before he took training, and he sounded very positive.
"I've also received a lot of good wishes from fans and I would like to thank them for their kind thoughts in wishing Gerard a speedy recovery.
"I will see him this afternoon, along with Gary, and we will pass on the fans' messages. I know they will mean a lot to him.
"We would ask everybody to respect his privacy and that of his family at this time."