The 38-year-old model-and-broadcaster - who previously dated Hollywood movie star George Clooney - was thought to be sufferingwith the flu when she turned on the Christmas lights in Bond Street, London, last Thursday (18.11.10) but was later diagnosed with the disease.
A friend said: "Lisa was feeling rotten for a good few days. On Thursday night she switched on the lights and she was clearly unwell and struggling to hold it together.
"She had flu symptoms, major aches and pains and a migraine. She then had to go on to host a dinner. But after that she just fell apart and she went to the doctor and was immediately checked into hospital.
"They then diagnosed her withviralmeningitis."
Although Lisa has been rushed to hospital, the disease is the less serious form of meningitis, meaning she should be out of hospital by the end of the week.
However, she may not recover completely until Christmas, meaning she will be unable to present her Capital Radio morning show with Johnny Vaughan.
The friend added to the Daily Mail newspaper: "Lisa`s unlikely to be returning to the breakfast show next week. She is now on the mend, but she`s still in hospital. The doctors have warned her this takes a long while to recover from."
The British beauty - who is currently the face of Marks & Spencer`s festive advertising campaign - has been replaced by Saturdays singer Frankie Sandford on her radio show.