Fewer people are having the Measles vaccine in Guernsey.
In recent years the number of islanders being protected against the disease has fallen from 96% to 92%.
The viral illness, which causes a fever and a red rash, is highly infectious and common in children. Complications from the disease include pneumonia and inflammation of the brain called encephalitis, which can be fatal.
Before modern immunisation programmes, hundreds of thousands of people caught Measles in the UK with around a hundred dying each year.
Guernsey's Director of Public Health says the situation is a worry.
Parents are being urged to make sure their families are vaccinated before travelling during the Easter holidays.