Bronmeirion Surgery worker had measles on duty as Porthmadog cases reach 42
More than 60 people are being offered the MMR vaccination after a health worker was identified as being infectious while on duty.
Health officials say the worker had seen patients at Bronmeirion Surgery in Penrhyndeudraeth while infectious.
Meanwhile an outbreak of the virus in the nearby Porthmadog area has reached 42 cases.
Most are connected to Ysgol Eifionydd school, but the heath care worker and 11 other cases have no direct link.
Public Health Wales said it was notified on Thursday that the worker at Penrhyndeudraeth had developed the illness, and had been at work for two days while infectious.
It is possible to be infectious for four days before the rash appears, and the health care worker would not have known they were infectious, said a Public Health Wales spokesperson.
As a precaution, the Public Health Wales Health Protection Team, is contacting the people who have been in contact with the worker by letter or telephone to inform them.
Where appropriate they will be be offered an immediate MMR vaccination.
Other patients who visited the surgery this week are not at increased risk of catching measles and the health care worker is recovering at home.
Dr Judy Hart, consultant in communicable disease control for Public Health Wales said most people born before 1970 are immune to measles, because they will have been exposed to the infection in childhood.
"People younger than this may not have been exposed to measles and so they may be at risk unless they have received two doses of the MMR vaccine," she said.
"MMR is a safe and effective vaccine that protects children from the most severe and unpleasant viral-rash illness of childhood," she added.
Parents are being urged to get their children vaccinated especially as large numbers of children will be coming together for the Urdd Eisteddfod in coming weeks.
Dr Hart said: "If you or your child is unwell and you suspect it is measles you should contact your GP by telephone or NHS Direct Wales on 0845 4647.
"Your child should not attend school or nursery and you should not attend work for five days after the rash starts," she added.