Three hospital wards were closed when a sickness bug broke out at the Royal Sussex in Brighton.
The wards were closed to new admissions after an outbreak of norovirus.
Patients were struck down with sickness and diarrhoea as the illness, which typically breaks out in winter, closed hospital wards for the first time since January.
On Wednesday the hospital's Acute Medical Unit (AMU), where patients are sent to be assessed after being admitted to A&E, was closed to new admissions when there was an outbreak.
The short-stay Chichester ward and the Vallance ward, which specialises in elderly care, which are both in the ageing 19th century Barry Building, were also closed after patients admitted to the hospital through the AMU were moved onto those wards. The AMU re-opened yesterday.
A spokeswoman for the Brighton and Sussex University Hospitals NHS Trust, which runs the Royal Sussex, said: “We're handling it using all the usual measures and when the areas are free of symptoms for 48 hours we deep clean and re-open the wards.
“Three wards have been affected over the last couple of days, one of which has already re-opened and the other two are under observation and closed to new admissions.”
Yesterday the trust's website warned anyone feeling unwell with symptoms of sickness and diarrhoea people to stay at home, drink plenty of water and not to visit friends or relatives in hospital until 48 hours after the symptoms have eased to reduce the risk of spreading the infection.
Between April 2012 and April last year, hospitals run by the trust dealt with 37 outbreaks of the highly-infectious virus.
A total of 341 patients staying at sites run by the trust had fallen ill with suspected norovirus and another 78 members of staff also caught the virus over the same period.
In the same month, two wards were shut at Eastbourne District General Hospital as they battled to control the bug. After a clean bill of health over the summer norovirus struck again in October 2013.
In the same month two patients on Chichester Ward at Royal Sussex were treated for norovirus and 11 days later six patients on Bailey Ward at the same hospital were struck down with the infection.