NHS bosses are advising parents to watch out for signs of measles following an outbreak in West Yorkshire.
The highly infectious disease, which can affect adults as well as children, has broken out in Leeds and Liverpool.
And local NHS organisations are on the look out to see if the illness has spread any further.
While immunisation has reduced the prevalence of the disease, an outbreak of measles can occur and spread very quickly.
NHS Choices has recommended that parents have their children immunised as soon as possible, if they have not had the vaccine already.
Symptoms of the disease include sore, watery eyes, fever, coughing, aches, a rash of red-brown blotches and a general unwell feeling.
The symptoms can be worse in teenagers and adults.
But two doses of the MMR vaccine - which also protects against mumps and rubella - can protect against measles.
The vaccine is free, safe and available from GPs.
Anyone with measles is advised to stay at home and phone their GP or NHS 111 for advice, to avoid spreading the illness.
Dr Joanna Cartwright, consultant in health protection at PHE, said: “Those affected in Liverpool are all recovering but it’s important to be aware that measles can be a very serious illness and lead to severe complications, especially in people who are particularly vulnerable or have other health conditions.
“The free MMR vaccine is a safe and effective way of protecting against measles, as well as mumps and rubella.
“It’s particularly important for parents to take up the offer of MMR vaccination for their children when offered at one year of age and as a pre-school booster at three years and four months of age.
"If children and young adults have missed these vaccinations in the past, it’s important to take up the vaccine now from GPs, particularly in light of the recent cases in Leeds.
“Check your child’s Red Book to see if they’ve received MMR vaccinations as scheduled, or check with your GP surgery if you’re unsure.
“Measles is extremely infectious to anyone who may not be immune. If you think you could have measles, it is really important to stay away from areas where you could come into contact with lots of other people – especially vulnerable patients in hospitals, care homes or other settings.”