People in western Sydney are being urged to watch out for measles symptoms after a baby was diagnosed with the highly infectious respiratory illness.

NSW Health says the baby spent time at Parramatta and Westmead on Monday while infectious.

The baby, who is too young to be vaccinated against measles, acquired the infection while in India before returning to Sydney.

NSW Health's Christine Selvey is urging people who may be susceptible to measles, and anyone born in or after 1966 who has not received two doses, to get vaccinated by Thursday.

"Measles is a highly contagious infection and the most vulnerable are infants under 12 months, who are too young to be vaccinated against it,' Dr Selvey said on Wednesday.

People who are not fully vaccinated and those with a weakened immune system are also vulnerable with the infection easily spread through coughing or sneezing.

Symptoms include fever, sore eyes, runny nose and a cough followed three or four days later by a red, blotchy rash spreading from the head and neck to the rest of the body.

It can take up to 18 days for symptoms to appear after exposure and anyone who develops signs should call their GP to ensure they don't spend time in the waiting room with other patients, Dr Selvey said.

The measles-mumps-rubella vaccine is free for anyone born during or after 1966 who hasn't already had two doses.

Anyone who was at three western Sydney locations below on Monday should be alert for measles symptoms until April 14.

* The Children's Hospital at Westmead emergency department waiting room between 12pm and 9:30pm

* Argyle Street Medical Centre at Parramatta between 9:15am and 10:00am

* Westfield Parramatta between 9am and 10am