A total of 11 patients at the Royal Alexandra Hospital in Paisley are currently showing symptoms, including diarrhoea and vomiting, thought to be linked to norovirus.
The wards are closed to new admissions, but one ward reopened on Wednesday.
Six patients have died since last week but hospital chiefs say they all had underlying medical conditions which caused their deaths.
They had all also contracted norovirus.
Dr Teresa Inkster, NHS Greater Glasgow and Clyde consultant microbiologist, said: "Norovirus is particularly prevalent in both the community and in healthcare settings.
"Typically it has a sudden onset and can spread quickly. Most people will recover within 24 or 48 hours."