Four backpackers staying in Perth have been diagnosed with measles, sparking an alert.

Four backpackers visiting Perth have been diagnosed with measles.

The first traveller contracted the illness after a trip to Bali and three more people have since been diagnosed.

All four were staying in accommodation in Northbridge.

WA Health says there are a further two people who are suspected of having measles and who have been tested.

The public is warned they may have been exposed to the risk of measles if they frequented Northbridge between July 1-7, and July 13-20.

WA Health warns people could have been exposed if they went to Spinner's Backpackers Hostel in the first and third weeks of July, Royal Perth Hospital emergency on Thursday, July 7, the Brass Monkey Hotel on Friday, July 15, or Connections Nightclub on Saturday, July 16 into Sunday morning.

Anyone who went to the Northbridge Medical Centre at the following times may also have been exposed:

• Monday July 4 between 4pm-5pm
• Saturday July 16 between 10am-2pm
• Monday July 18 between 12.30pm-6pm
• Tuesday July 19 between 3pm-4.30pm

Measles is a highly contagious viral illness that can cause serious disease.

Early symptoms include fever, cough, runny nose and sore eyes, followed by a red blotchy rash about three days later.

It is contagious for about four days before and after the development of the rash, which usually starts on the face and spreads to the rest of the body.