A measles outbreak has been declared after six people required hospital treatment.
The Health Protection Agency (HPA) said there were 13 confirmed and 16 suspected cases on Merseyside, ranging in age from nine months to adults in their 20s.
Of the six people who required hospital treatment, four are children and two are adults.
This compares to just one confirmed case on Merseyside in the first six weeks of last year and one in the corresponding period of 2010.
The majority of the confirmed and suspected cases are in Liverpool, with one small family cluster of one confirmed and two suspected cases in St Helens.
Parents are urged to ensure that their children are fully vaccinated.
Dr Roberto Vivancos, a consultant with the HPA, said: "Measles is circulating in the community. We have alerted GPs and asked them to notify us of any cases they come across. We are also asking parents and GPs to do all they can to ensure that children are fully protected by MMR vaccine."
The most common symptoms of measles are fever, cough, sore eyes and a rash that develops over three to four days, beginning with the face and head and spreading down the body.
People with symptoms are advised to stay away from school, nursery or work until four days have elapsed after the development of a rash.
They are also asked to avoid contact with pregnant women and to telephone their GP for advice before attending the surgery.