Three wards have been closed off at Leicester Royal Infirmiary (LRI) after fourteen cancer pateints have contracted H1N1, commonly known as swine flu.
Three other patients at Leicester's Glenfield hospital, two adults and a child, are also being treated for the virus, Leicester Mercury reports.
Once H1N1 was discovered at LRI, the patients were quickly isolated and given antiviral drugs.
Dr Philip Monk, a communicable disease consultant with Public Health England said: "On Monday, it was thought three people on the wards might have flu, and because these patients have very little immunity, it was decided to swab all patients and 14 were confirmed as positive."
The Telegraph reports that cleaning has been increased in the affected areas and clinics have been set up to vaccinate hospital staff.
Visitors with cold and flu symptoms such as a "cough, runny nose or high temperature" are being asked to stay away from Leicester hospitals to protect patients.
Dr Monk said the number of cases across the East Midlands and the country have been increasing over the past three weeks.
He added added: "This year's flu vaccination is a good match for H1N1, and is the best way of stopping it from spreading.
"People can be quite poorly if they get this flu, but if they have had it they won't get it again because they have built up an immunity."
The 2009 swine flu epidemic killed over 100 people in Mexico.
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