A leading child health expert has warned that an outbreak of measles on Merseyside is a "timely reminder" that Britain remains susceptible to the disease.
The Health Protection Agency said yesterday that Merseyside is suffering its worst outbreak of measles since the introduction of the MMR vaccine in 1988, with 214 confirmed cases since January.
Half the cases registered in the metropolitan area have affected children, with another 30% aged 15 or over.
Kim Mulholland, professor of child health and vaccinology at the London School of Hygiene and Tropical Medicine, said: "The current outbreak of measles in the Merseyside area is a timely reminder that Britain, like other European countries, remains susceptible to measles outbreaks.
"Over half of the more than 200 cases were under five years of age, which is not surprising given that more than 10% of British children under five are not vaccinated against measles.
"This provides more than enough susceptible children to sustain outbreaks, which will continue while immunisation coverage is inadequate and measles continues to circulate in other parts of the world."
Measles is a "very infectious" illness which spreads quickly among children and adults who are not vaccinated, and can lead to serious complications and, on rare occasions, death.
Symptoms include fever, cough, a runny nose, red eyes and a red rash.
At least 39 of the confirmed cases on Merseyside have needed treatment in hospital and there are 92 probable cases still being investigated.
The worst-hit areas have been Liverpool and neighbouring boroughs such as Knowsley and Sefton.
People with the disease are advised to avoid contact with others, especially pregnant women and children as they are more vulnerable to infection.
They are urged to stay away from schools, nurseries and work until at least four days after the onset of the rash.
Before attending a medical centre they should telephone their GP in advance so arrangements can be made to prevent others being infected.