This year is on course to be the worst year for measles outbreaks in two decades.
Already, in the first three months there have been 587 cases in England. With 886 cases in the Swansea outbreak, 2013 has already seen over 1,400 cases in England and Wales - in the whole of last year, there were only around 2000 confirmed cases.
Professor David Salisbury, the Department of Health's most senior doctor on immunisation, told me:
I don't think we've got an emergency - at the moment. We need to prevent an emergency.
He reminded me that last year, 20,000 cases were logged in an outbreak in France:
We could have thousands of cases if we don't get ahead of this and prevent them.
He pointed to the message from Swansea:
I think what we've seen so clearly from Swansea is the risk - and the risk could indeed spread to England and we are acting ahead of that risk.
David Salisbury has been the Government's top adviser on immunisation for many years.
He's a wily old fox. If he thinks the risk of a 'measles emergency' in England is a real possibility, as he clearly does, I believe him.
Watch more of my interview with him below.
The problem arises because of children who weren't vaccinated at the time of the scare stories about the MMR jab and autism.
Those stories were baseless. But many parents were, understandably, panicked and didn't have their children immunised.
Those children are now between 10 and 16, and that's the age group that's bearing the brunt of the epidemic in Swansea.
In England, London is a big worry.
In 2006/7 nearly 50 per cent of children didn't get the full course of two MMR jabs. But other areas are showing high measles rates - 175 so far this year in the North East and 179 in the North West.
If you think measles is a "mild" childhood disease, you're wrong.
Nearly 1 in 5 people with measles this year had to go to hospital. 15 suffered suffered complications like pneumonia. If you aren't impressed by statistics, look at your local paper. I'll bet that in the last year, it's had a story about a baby that nearly died.
And in Swansea, one 25-year-old who died was infected with measles, though it's not clear yet whether it played a part in his death. Measles can be very serious - especially for babies too young to be vaccinated and for older people in the teens and twenties.
That's why Public Health England is starting a campaign to vaccinate teenagers who are unprotected against measles and who could pass on the infection to people like kids with leukemia who can't be vaccinated and are therefore vulnerable.
Dr Paul Cosford of Public Health England told me there was now a chance to eliminate the condition.
My advice, if you want it, is very simple: get your kids vaccinated to protect them and others. Almost everyone can have an MMR jab. Why would you not?