Norovirus is normally brought into hospitals by patients, visiting relatives or even staff.
It is also linked to outbreaks at schools and is responsible for many hospital admissions for young children.
The elderly are also vulnerable because they can become dehydrated quickly and end up requiring more hospital treatment.
It is extremely contagious and is often spread by touching the same doors or stair rails as someone with the virus.
Experts also say that when a patient vomits, there is a strong chance anyone else in the room will be infected.
It puts the NHS under huge strain because hospitals have to close wards - which limits the number of patients that can be admitted.