A baby boy is among three Melburnians infected by a measles outbreak.

The outbreak occurred between September 6 and 13 and experts are warning that more people may have been exposed to the infectious illness.

At least one of the patients contracted the illness locally, while the the other case may have been infected overseas.

Deputy Chief Officer Dr Brett Sutton warned Victorians to be aware of potential signs of the illness.

“The source of the infection is unknown at this point. This means there may already be secondary cases in the community which have not been diagnosed,” Dr Sutton told The Herald Sun.

The patients attended train stations between Armadale and Southern Cross stations, Collins street, Chadstone shopping centre, IKEA Richmond and Ballarat.

“People who attended these locations between September 6 and September 13 may be at risk of developing measles,” Dr Sutton said.

People at risk include children and adults born in or post 1996, who haven’t received two doses of a vaccine to protect themselves from the illness.

The symptoms of the illness include runny eyes, a runny nose, a severe cough, high fever and a rash.

“Measles is a virus infection, it’s an infection, that initially hits the respiratory tract, the throat, the eyes, the nose and children have a high fever for five or six days,” Dr Samuel Katz of Duke University said in a United Nations Foundations video.

“They [people infected with measles] develop runny eyes, which we call conjunctuvitus, runny nose, which we call coryza, and cough because the virus is down in the respiratory tract.”

“So the three C’s. Cough, coryza and conjungtavitus, and high fever and then the rash appears after that has gone on for a week.”