“It’s very infectious and strikes in winter because people are indoors and it spreads more easily,” Hawkey said.
Elizabeth has ruled since 1952 and is Britain’s second-longest serving monarch, beaten only by Queen Victoria in terms of the number of years spent on the throne.
Elizabeth’s husband Prince Philip, 91, has had several hospital stays, but Elizabeth has rarely let sickness get in the way of her still-busy schedule.
About five months ago, she cancelled an engagement due to a bad back. The spokeswoman, who demanded anonymity because palace rules do not let her go on the record, said the last time Elizabeth was hospitalized was in 2003 for a knee operation.
The queen has undertaken a number of engagements over the past week. On Tuesday, she met the new archbishop of Canterbury, Justin Welby, at Buckingham Palace, and on Thursday she presented a host of British Olympians, including track and field star Jessica Ennis, with honors during an investiture ceremony.
Ingrid Seward, the editor of the Britain’s Majesty Magazine, said that the queen “probably agreed to be hospitalized in order to get better quickly.”
“Everybody will want to be wishing her a speedy recovery,” she told Sky News television.
That includes British Prime Minister David Cameron, whose office said he passed on his best wishes to the queen.