A head teacher has written to parents to remind them to make sure their children are immunised after a measles outbreak at his school.
Jim Adams wrote to parents after three cases were confirmed at Hobart High School, Loddon, Norfolk.
"Measles can be a very serious illness and it is also highly infectious," he wrote.
Dr Louise Smith, director of public health for Norfolk, said it was "never too late" to be immunised.
In his letter to parents, dated 10 July, Mr Adams said the three children were recovering but warned there could be further cases at the school.
"If you are not aware of your child having received two doses of MMR [the measles, mumps and rubella vaccine], please contact your general practitioner to ensure that your child is protected," he wrote.
Mr Adams told the BBC: "We contacted parents and carers in order to make them aware and to share advice from Public Health England.
"There have been no subsequent reports of any further cases."
Dr Smith described measles as a "nasty illness".
She said false stories dating back about 18 years regarding the safety of the MMR vaccine had led some parents not to have their children immunised.
That had led to outbreaks around the country as teenagers moved away from home, mixing with new groups of people.
"We have always had some people who worry about vaccination," she said.
"Our message is that since this vaccine came in the 1980s, millions of children have been vaccinated and overwhelmingly the evidence is that it's safe.
"The only people who can't be given the jab are pregnant women. For everyone else, it's never too late to have the vaccine."
Public Health England said there were 913 confirmed cases across the country between January and October 2018, compared to 259 in 2017.