More than 100 people have, or are suspected to have, measles, in one of the worst outbreaks in the North East in recent years.
The Health Protection Agency (HPA) said the disease was mainly spreading among unvaccinated schoolchildren and it was "seriously" concerned.
Since September there have been 49 suspected and 56 confirmed cases, compared to just 18 confirmed in 2011.
Almost a quarter of those affected have been hospitalised, the HPA said.
'More cases likely'
Parents are being urged to make sure their children have the required two doses of the MMR vaccine - the first at 12 months and the second dose at around three years and four months.
HPA North East spokeswoman, Julia Waller, said: "This measles outbreak is very serious and we are likely to see many more cases before it's over.
"The sad thing is that most, if not all of these cases could have been avoided if people had been up-to-date with MMR vaccination.
"There are still too many children and young adults who were not vaccinated.
"People with measles could also be a risk to those who are not able to protect themselves such as babies who are too young to be vaccinated."
Earlier in January the HPA issued similar guidance in the North West after there were 35 confirmed cases since September.
The most common symptoms are fever, cough, sore eyes and a rash that develops three or four days after the onset of illness, starting with the face and head and spreading down the body.
There is no treatment for measles and in very rare cases it can be fatal.