A mumps outbreak that was first declared on the University of Alberta campus was expanded Tuesday to include the entire Edmonton area.

Twenty-four of Alberta's 51 confirmed mumps cases are in Edmonton. Only eight are related to the University of Alberta outbreak.

Dr. Joanna Oda, the medical officer of health for the Edmonton zone of Alberta Health Services, says an outbreak is declared when the number of cases that are not related to travel are higher than usual.

"Last year we saw eight cases of mumps in the entire province for the entire year so the fact that we are at the end of March and we've seen 24 cases that were locally acquired in the Edmonton zone, that's why we're calling this an outbreak," she said.

Last month, officials said four University of Alberta students had confirmed cases of the virus.

Oda said there are outbreaks in other provinces and in the United States. She said the first Alberta case was contracted in Manitoba which had a mumps outbreak in November.

Alberta Health Services is offering two free doses of mumps vaccine to anyone in the Edmonton area who was born in or after 1970, and who hasn't had the proper recommended dosage.

Oda said some people may have only had one dose of the vaccine, as was the practice from the 1970s to the 1990s. Two doses give the best protection.

Mumps is a contagious viral infection, which often causes swelling and pain in the glands of the jaw. Some people don't experience swelling but may feel like they have a bad cold or the flu instead.

Most people recover from mumps without long-term impact but in some cases it can cause serious complications affecting the brain, testicles, ovaries or pancreas. It can also cause deafness in children.

Oda said people should avoid sharing drinks and utensils as the virus is spread through saliva.

People born before 1970 are considered immune because mumps was common then. Anyone who has received two doses of mumps vaccine doesn't need a backup.