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."
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Pictures Of The Day: 3 May 2012
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Palestinian youths lower their heads as a fellow protester throws back a tear gas canister at Israeli troops during clashes outside Ofer military prison near the West Bank city of Ramallah on May 2, 2012, following a demonstration in solidarity with prisoners held in Israeli jails. Clashes erupted for the second day in a row between stone-throwing youths and the Israeli army near the military jail in the occupied West Bank. TOPSHOTS AFP PHOTO/ABBAS MOMANIABBAS MOMANI/AFP/GettyImages
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A vendor covers his head with a basket from the sunlight in Bangkok, Thailand, Thursday, May 3, 2012. (AP Photo/Sakchai Lalit)
A model wearing designs by Jenny Lee stands in a display made of natural props including leaves and branches from native Australian trees, during Australian Fashion Week in Sydney, Thursday, May 3, 2012. (AP Photo/Rob Griffith)
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Artist Henry Bruce looks up at his largest, possibly most controversial art project to date, in the grounds of the Delamore Estate. As part of the art and sculpture exhibition at Delamore Henry has painted a massive skeletal 21 metre high oak tree fluorescent pink to celebrate the beauty of nature and the evolving life of trees.Picture date: Wednesday May, 2, 2012. Photo credit should read: Ben Birchall/PA Wire
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File photo dated 12/04/12 of Sotheby's staff members hanging The Scream, by Norwegian painter Edvard Munch at the auction house's central London offices. The only privately-owned version of the masterpiece fetched a record-breaking 119.9 million US dollars (£74 million) at auction.Issue date: Thursday May 3, 2012. The famous image, which features a haunted figure in front of a red backdrop, surpassed estimates to become the most expensive work of art to be sold at auction following the sale in New York. See PA story SALE Scream. Photo credit should read: Lewis Whyld/PA Wire
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Monkey Swimmers A group of monkeys cool down in the heat by taking a swim in the water in Guiyang, China. They enjoy hanging from the trees and playing around by leaping into the water. China - May 2012 Credit: WENN.com
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Fabrice Muamba of Bolton Wanderers shows his emotion after being introduced to the crowd prior to the Barclays Premier League match between Bolton Wanderers and Tottenham Hotspur at the Reebok Stadium on May 2, 2012 in Bolton, England. (Photo by Michael Regan/Getty Images)
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A tern feeds on a marsh near the village of Smolichi, 160 km (100 miles) south of Minsk, Belarus, early Thursday, May 3, 2012. (AP Photo/ Sergei Grits)
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Newcastle Unied's Papiss Cisse celebrates his goal during the Barclays Premier League match at Stamford Bridge, London.Picture date: Wednesday May 2, 2012. See PA story SOCCER Chelsea. Photo credit should read: Rebecca Naden/PA Wire.