The first measles case has now been confirmed in Cumbria, prompting a further warning to parents.
The disease has been spreading across northern England since before Christmas, mainly in the Merseyside area.
But it was gradually moving towards Cumbria, and now the first case has been confirmed by Public Health England (PHE).
This is in Barrow during the past week.
Measles is a potentially serious illness that can lead to permanent disability and even death.
Until this week the closest confirmed cases were in Lancashire and the north east, though it is highly contagious.
Now there are fears it will spread across Cumbria.
It comes after Public Health England wrote to schools and nurseries across the county warning of a potential outbreak.
Now parents and teachers are again urged to be on high alert.
Parents who have not had their children vaccinated are urged to ensure they get the MMR jab urgently.
Adults should also ensure they are vaccinated.
This protects against measles, mumps and rubella (German measles).
Mumps cases have also been recorded across the county in recent months. Another was confirmed in Carlisle in the past week.
The MMR jab is usually given in two doses, the first at the age of one and a pre-school booster just after children turn three.
There are also ongoing outbreaks in other parts of Europe, including Romania, Italy and Germany. PHE is therefore warning that anyone travelling to these countries is at particular high risk.
Mark McGivern, health protection consultant at Public Health England North West, said: “Measles is a very infectious virus and can spread rapidly among communities, such as schools, if people have not been fully immunised.
"While most people who catch measles will recover completely within a couple of weeks, it's important to remember measles can be a very serious illness that can leave permanent disability, and occasionally even kill.
"MMR not only protects them, but also limits the chances of the virus spreading more widely, for example to babies who are too young to have the vaccine and to adults who may be more vulnerable to the disease, for example because of cancer treatment."
Because measles is so infectious, anyone with symptoms is advised to stay at home and phone their GP or NHS 111 for initial advice.
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