Four hospital wards have been closed to visitors due to a norovirus outbreak.
Royal Berkshire Hospital in Reading said six patients had tested positive for the winter sickness bug and five further cases were being investigated.
Health bosses said it was asking people with "symptoms of sickness or diarrhoea or are feeling unwell" to avoid going to the hospital for "at least 48 hours after their symptoms have gone".
The hospital said it would give updates on the virus "over the next few days".
What is norovirus?
Sometimes known as "winter vomiting viruses", noroviruses are the most common cause of stomach bugs in the UK, affecting all ages.
It is estimated that between 600,000 and one million people in the UK become infected each year.
About 12 to 48 hours after becoming infected, the virus causes sudden onset of nausea followed by vomiting and diarrhoea.
The illness is not generally dangerous and most people make a full recovery in one or two days.