The health board also said it was having difficulty coping with capacity at the RoyalInfirmaryofEdinburgh because of rising population levels in Lothian.
Relatives and friends of patients are being advised not to visit the wards at the RoyalInfirmaryofEdinburgh in an effort to stop the outbreak spreading.
Officers are keen to trace anyone who may have been in the area of the RoyalInfirmaryofEdinburgh at about 22:00 on Friday, and who may have seen the men.
Robin Harper (Green) asked the first minister to suspend suspend the contract of food supply to Edinburghroyalinfirmary so patients receive a better standard of service.