Two cases of norovirus - usually associated with the winter months - have been confirmed at Watford General Hospital.
And another two patients are showing signs of the highly contagious infection.
Medical director Dr Mike Van der Watt said: “Norovirus is most commonly found during winter months so it is quite unusual for us to see patients with this infection at this time of year.
“As soon as we became aware of this issue, we immediately put all necessary precautions in place to help stop the potential spread of infection across the hospital, including isolating patients showing symptoms as well as any patients who they may have come into contact with.
“There is no specific cure for norovirus and the best option is to let it run its course and for everyone to observe basic rules of good hygiene.”
There are currently no plans to restrict visiting, but anyone who is visiting patients infected with the winter vomiting bug is being asked not to see other patients elsewhere in the hospital.