Vaccination numbers in Donegal are too low to prevent the spread of measles.
While the majority of children are vaccinated, rates of those receiving the MMR vaccine in the county has dropped recently to 88%.
Rates need to be above 95 % in order to stop the infection from being passed-on.
Three cases of the infection have now been confirmed in Donegal and there are two more probable cases awaiting confirmation from laboratory testing.
Measles is very infectious and the illness can have serious complications, especially for infants under 12 months and those with weakened immune systems.
Public Health Doctor Laura Heavey says vulnerable groups of children need to be protected.