Measles outbreak in NSW prompts warning as third person diagnosed in less than a week.

A child has become the third person diagnosed with the measles in less than a week in NSW.

NSW Health on Wednesday night said the child had presented to Royal North Shore Hospital after becoming infected.

A spokeswoman said staff are still confirming details about the latest case with more information expected on Thursday.

It’s the third confirmed case in less than a week after NSW Health earlier on Wednesday said a visitor from the ACT was diagnosed with the infection.

The ACT resident was infectious while visiting the Central Coast and Sydney’s northern suburbs between December 26 and 30.

The person visited Thornleigh McDonald’s, Jasmine Cafe at Umina Beach and made several visits to Deepwater Plaza in Woy Woy and Umina Beach Shopping Centre.

It follows another confirmed case on December 29 when a young adult from Sydney returned from Thailand and was diagnosed with measles.

Communicable diseases director Vicky Sheppeard says anyone in those locations at the same time should watch for symptoms.

“The time from exposure to the disease to the onset of symptoms is typically about 10 days but can be as long as 18 days so people should be alert to symptoms until mid-January,” Dr Sheppeard said in a statement.

The infection is highly contagious and can spread through coughing or sneezing.

Measles symptoms include fever, sore eyes 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.

Infants under 12 months, who are too young to be vaccinated, and young adults are most likely to be susceptible to the infection, Dr Sheppeard said.