About 40 patients and staff at Eastbourne District General Hospital have been struck down with an infectious stomach bug.
The outbreak of gastro-enteritis has hit several wards.
The hospital is already at full stretch following the Christmas and New Year period. Some non-urgent operations have had to be cancelled.
Hospital staff today asked the public for their help in containing the spread of the bug.
People visiting friends and relations in hospital are being encouraged to wash their hands on leaving a ward.
Gastro-enteritis is not dangerous in itself, although it is distressing for patients who may catch it.
But it can be debilitating for patients who may be elderly or who are weak because of another condition.
Symptoms are diarrhoea and vomiting, sometimes accompanied by headaches or fever.
Staff, including the hospital's infection control team, are working to contain the outbreak by isolating or segregating affected patients and ensuring that anyone who has contracted the virus remains off work for 48 hours.
Assistant Chief Nurse Mandy Rann, said: "If visitors experience any symptoms, please refrain from visiting the hospital for at least 48 hours."
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