More than 200 cases of measles are confirmed on Merseyside in the largest outbreak in the area since 1988, the Health Protection Agency has said.
There are 210 confirmed cases, 39 of which needed hospital treatment. Ninety two cases are under investigation.
About 50% of the cases are in children under five years old.
The outbreak, which is concentrated on Liverpool where there are 125 confirmed cases, is the largest since the MMR vaccine was introduced.
There are 29 confirmed cases in Knowsley, 22 in Sefton and seven in Wirral. Further cases have been confirmed in Cheshire, Halton and St Helens.
More than 30% of cases have been in people aged 15 years or over.
Dr Roberto Vivancos, a Health Protection Agency consultant, said: "It's 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 is a very infectious illness that spreads rapidly amongst children and adults who are not protected by MMR vaccine.
"It is also a serious illness that can lead to serious complications. On rare occasions, people die from measles.
"It should not be treated lightly, but it is an avoidable illness and we strongly advise parents to ensure that their children are vaccinated."