Toronto public health officials are warning people in their teens, 20s and 30s to make sure they are properly vaccinated against measles after a recent outbreak of the illness in the city.

On Thursday, officials confirmed four new cases in the past few days. It brings the number of cases in the Greater Toronto Area to nine. There are also outbreaks in other parts of the province and in the United States.

The people affected either live or work downtown, said Dr. Barbara Yaffe, director of communicable disease control for Toronto Public Health. The agency managed to trace two points of contact — the TD Waterhouse building and the Scotia Tower, which are connected by the city's underground pathway system.

Yaffe said anyone born before 1970 probably had measles in childhood and therefore won't get it again.

"Everyone else should ensure they have had two doses of measles vaccine for full protection, especially as we know measles is circulating in Toronto,” she said.

Anyone born in the 1970s up until 1995 likely only had one vaccination shot, she added.

Measles is a highly contagious and airborne virus that is also spread by droplets and direct contact with nasal and throat secretions of an infected person. A person with measles can infect others from four days before to four days after the onset of rash.

People who get measles generally recover fully after 10 days of sickness, but the effects can be more severe for infants, the elderly and pregnant women.