Health authorities are warning anyone who has been in Leichhardt's busy shopping district and library recently to be on the lookout for symptoms of measles.

The Sydney Local Health District has issued an alert on its Facebook page, saying an infected person visited MarketPlace Leichhardt, Norton Plaza and the library, in Sydney's inner-west, several times between July 15 and 24.

A child with a bad case of measles.

The alert said the symptoms of measles include sore eyes, a fever and a cough, followed by a rash a few days later.

It said the measles virus was highly contagious and spread through the air.