Cases of measles are rising, figures show, amid continuing poor uptake of the MMR vaccine.
Measles can cause very severe symptoms
Catch-up campaigns to immunise schoolchildren are being carried out around the country in a bid to prevent an epidemic.
What is measles?
Measles is a highly contagious infectious disease characterised by a high fever, a rash and generally feeling unwell.
Around one in every 15 children have complications which include chest infections, fits, encephalitis (swelling of the brain), and brain damage.
In very serious cases, measles can be fatal.
How many cases of measles have there been?
In 2008, there were 1,348 measles cases compared with 990 in the whole of 2007.
It is the first time the number of cases has topped 1,000 since 1995, the Health Protection Agency reports.
In 1998 there were just 56 cases in England and Wales.
What has caused the increase?
Experts say measles is spreading more easily because of low uptake of the MMR vaccine over the past decade.
MMR is a combined vaccine against measles, mumps and rubella, three common infectious diseases of childhood.
It was introduced in the UK in 1988 to replace single vaccines for each disease.
But there are growing number of children who are unprotected - about one in four have not had both MMR doses.
It is estimated this could result in between 30,000 and 100,000 cases of measles in England alone.
Why have MMR vaccination rates declined?
In 1998, a study published in the respected journal The Lancet raised the possibility that the jab may be linked to autism and bowel disease.
The paper and the media furore that followed it prompted many parents to decide against having their children vaccinated with the three-in-one jab.
Complications of measles:
One in 2,500-5,000: Death
One in 10: Hospital treatment
One in 1,000: Meningitis
Some opted to have their children vaccinated using single vaccines for each disease. However, others decided against having their children vaccinated against these diseases at all.
No research has been published to back up claims that it may be linked to autism and bowel disease.
And many studies that have examined the safety of MMR since 1998 have all concluded that the three-in-one jab is safe.
The Lancet, which published the controversial MMR paper in the first place, has since publicly announced it should never have printed it.
Mumps, measles and rubella are all serious diseases, particularly measles which can be fatal.
The UK government and the vast majority of scientists insist that the MMR, which is used in many countries around the world is safe.
What is the catch-up campaign?
Infants are vaccinated at 13 months and once again before they start school.
The first dose of MMR does not protect everyone against measles and mumps hence the need for a second dose.
But only 75% of children have had both doses. In August, the government launched a campaign to raise MMR vaccination rates in England to make sure as many children as possible were protected.
The Department of Health has asked primary care trusts (PCTs) to offer the jab to all children up to the age of 18 not already fully protected.
In the catch-up campaign, the two doses are usually given three months apart but can be given with a gap of just one month.
Some schools have also arranged to have pupils vaccinated to try and prevent further measles outbreaks.
Parents who wish to have their child vaccinated are advised to contact their GP.