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Stephen Hawking, shown here at Cape Canaveral, Fla., in April 2007, has been fighting a chest infection for several weeks. ((Peter Cosgrove/Associated Press))

Physicist and mathematician Stephen Hawking, one of the world's most famous scientists, is "very ill" in hospital, the University of Cambridge said Monday.

Hawking, a professor who teaches at the university, was taken to Addenbrooke's Hospital in Cambridge by ambulance on Monday, the university said.

The university told The Associated Press Hawking had been fighting a chest infection for several weeks.

"He is undergoing tests," said Gregory Hayman, the university's head of communications.

"He has been unwell for a couple of weeks."

Hawking's impact

Stephen Hawking was first diagnosed with amyotrophic lateral sclerosis (ALS) in 1963 at the age of 21, but it wasn't until three years later that he completed his PhD and began a distinguished career.

Here  are a few of his career highlights:

1966: Publishes paper "Singularities and the Geometry of Space-time," the first of several works where Hawking, along with mathematician Roger Penrose, showed Einstein's General Theory of Relativity implied space and time would have a beginning in the Big Bang and an end in black holes.

1974: In the journal Nature, Hawking's pivotal "Black Hole Explosions?" is published. Hawking makes the groundbreaking argument that black holes leak energy and eventually evaporate. This energy comes to be known as Hawking Radiation.

1979: Hawking is appointed to Cambridge's esteemed Lucasian Professorship of Mathematics — a post once held by Sir Isaac Newton. In 1979, Hawking also publishes Superspace and Supergravity .

1989: The best-selling A Brief History of Time: From the Big Bang to Black Holes is published. He later publishes The Universe in a Nutshell (2001) and A Briefer History of Time (2005).

2004: Hawking revises his Black Hole Explosions theory, suggesting the black holes release information before evaporating.

Hayman told the Associated Press later in the afternoon Monday Hawking was "now comfortable but will be kept in hospital overnight."

Hawking, 67, suffers from a disease that damages the motor neurons in his brain and spinal cord, a disease that  belongs to the same family of diseases as amyotrophic lateral sclerosis (ALS) — or Lou Gehrig's disease. He must use a wheelchair and speaks with the aid of a voice synthesizer.

He developed symptoms of the disease in the 1960s but has survived for decades with it and has, during that time, been one of the leading contributors to astrophysics, particularly with his work on black holes and his investigations into the origins of the universe.

He is perhaps best known for his book A Brief History of Time, one of the few physics books to become an international bestseller.

"Professor Hawking is a remarkable colleague. We all hope he will be amongst us again soon," said Prof. Peter Haynes, head of Cambridge University's Department of Applied Mathematics and Theoretical Physics.

Hawking had cancelled an appearance at Arizona State University on April 6 because of the illness.

Hawking is scheduled to visit the Waterloo-Ont.-based Perimeter Institute for Theoretical Physics this summer and give at least one public lecture.

Perimeter spokesperson John Matlock said on Monday it was too soon to know whether Hawking would be able to come in June-July as planned, but that thus far no changes to the schedule have been made.

With files from The Associated Press