A concourse at Chicago's O'Hare International Airport/AFP/Getty Images

Chicago O'Hare International Airport has issued a measles alert for air travellers, four days after a passenger with a confirmed case of the highly contagious virus passed through two of its terminals.

The Illinois Department of Public Health (IDPH), in its alert, warned travellers that an individual diagnosed with measles arrived at Chicago O?Hare International Airport on January 10.

The alert said the passenger, who ?was infectious on that day,? arrived on an international flight at Terminal 5 of the airport and departed on a domestic flight from Terminal 1, adding that the passenger ?may have traveled to other areas of the airport.?

?People may have been exposed to measles if they were at O?Hare International Airport between 6:30am and 1pm on January 10, 2018,? the statement continued.

The statement added that individuals who were vaccinated as children ?are not at high risk? but passengers who have not been vaccinated could start to develop symptoms as late as January 31.

According to the Centers for Disease Control and Prevention (CDC), measles is ?a serious, highly contagious disease? that is ?easily? spread through the air when an infected person coughs or sneezes. The disease can also spread through direct contact with an infected person?s nose or throat secretions.

Early symptoms of the disease can include a rash, fever, cough, runny nose and watery eyes. Infected individuals develop a high fever anywhere from 10 to 18 days after exposure and a rash that starts on the face or neck about 14 days after exposure.

In 2014, a record 667 cases of measles were reported to the CDC ? the greatest number since measles was declared eradicated in the US in 2000.