A hospital has closed three wards after a major swine flu outbreak affected 14 cancer patients.
Leicester Royal Infirmary is trying to contain the spread of the virus, which was responsible for the flu pandemic in 2009/10.
The H1N1 swine flu strain is one of the dominant flu strains this winter and the current winter flu jab offers protection against it.
Glenfield Hospital in the city is also treating three seriously ill patients with swine flu, who are receiving extracorporeal membrane oxygenation (ECMO) treatment.
They were brought to Leicester from other parts of the country.
Liz Collins, lead nurse for infection prevention at Leicester's hospitals, said: "We have seen an increase in the number of flu cases in February, in both the community and across our hospital sites.
"Fourteen patients on three haematology wards at the Leicester Royal Infirmary have developed symptoms that have been confirmed as flu. All necessary precautions were taken and these patients have been isolated to avoid an outbreak.
"We ask visitors who have cold and flu symptoms, such as a cough, runny nose or high temperature, to stay away from the hospital to avoid passing on their infections to our patients."
The wards that have been closed are wards 39, 40 and 41.
All 14 patients remain in isolation either in side rooms or on bays, a statement from the University Hospitals of Leicester NHS Trust said. All patients are currently being treated with antiviral medication.
Staff are following protocols for preventing the spread of infection, including wearing masks, gowns and gloves and washing hands regularly.
The statement added: "We will be working hard to vaccinate as many frontline staff as possible and will even be holding clinics in their workplaces.
"Wards 39, 40 and 41 have received increased cleaning by our facilities providers."
Dr Philip Monk, consultant in communicable disease control with Public Health England in the East Midlands, 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."
According to the latest Public Health England (PHE) data, 40 flu outbreaks were reported in the week ending February 7.
Twenty-two were in schools, with two testing positive for swine flu.
Sixteen outbreaks were in care homes and two in hospitals.
Across the week, there were 148 admissions to high dependency units and intensive care for people with flu, of which 73 had swine flu.
Data from GP surgeries suggests an increase in people seeking advice regarding flu but rates are still within expected levels for the season.