The World Health Organisation is concerned about a surge in the cases of measles in Europe this year.
The organisation says not enough children are being vaccinated for the disease, known mainly for its itchy rash, but which can be deadly.
Rebecca Martin, who runs the European office of the WHO's vaccine and preventable diseases program, says France reported about 5,000 cases of measles last year.
She says the country has already reported 4,937 cases between January and March 2011.
There has been one death due to measles in France this year.
"Belgium, which has to date recorded 100 cases in 2011 ... only reported 40 cases for the entire year of 2010," Dr Martin said.
She says people may have forgotten how severe measles can be, especially for children under five and adults older than 20.
"With people not seeing the disease over the years in the early 2000s, people have forgotten about the disease," she said.
"Children have not been immunised as regularly as before and we've had a build-up of children over time who have not had their immunisations.
"What this allows then is if the virus is introduced, it can quickly spread and move very fast."
Dr Martin says there are European communities who have not been vaccinated for either for philosophical or religious reasons.
She migrant populations are also vulnerable, with people moving across borders not getting their immunisations on time.
"This is something we had a meeting wit the European Union in early April to discuss about maybe developing an immunisation card that could be used across the region to help track children's immunisation to make sure that everybody's had two doses," she said.
Dr Martin says efforts to eradicate measles in Europe have been seriously setback.
