Health experts are urging people in Western Australia's South West and Great Southern to be on alert for measles as the number of community transmission cases climbs to its highest in almost three decades.

Measles is a highly contagious virus that causes a skin rash and fever, and can lead to serious and life-threatening complications.

There have been 11 cases reported in the South West and one in the Great Southern, with 18 exposure sites identified across both regions since October 22.

Twelve new exposure sites have been identified by the Department of Health south of Perth since October 27, with the most recent listed on November 4.

The exposure sites include emergency departments at the Margaret River Hospital and Albany Health Campus.

Other sites where members of the public may h

See Full Page