More than 200 confirmed cases of measles on Merseyside have contributed to the worst outbreak of the disease since 1988, according to the Health Protection Agency.
The independent body said there have been 214 laboratory-confirmed cases since January, while 92 probable cases are being investigated.
It is the worst outbreak in the metropolitan area since the measles, mumps and rubella (MMR) vaccine was introduced in 1988 and at least 39 of the confirmed cases have needed treatment in hospital.
Half the cases registered on Merseyside so far this year are children under the age of five, with another 30% aged 15 or over.
The majority of the confirmed cases have been in Liverpool and neighbouring areas such as Knowsley and Sefton.
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.
Dr Roberto Vivancos, a consultant with the Health Protection Agency in Cheshire and Merseyside, said: "It is obvious from these statistics that people who are not fully vaccinated are not just at risk themselves, but they pose an infection risk to others, such as defenceless babies and toddlers who are too young to be vaccinated.
"Measles should not be treated lightly, but it is an avoidable illness and we strongly advise parents to ensure that their children are vaccinated.
"Our advice to unprotected teenagers and young adults is to arrange vaccination through your family doctor. It is never too late to be vaccinated."