The MMR vaccine at the centre of the debate
Here one parent tells how his son - who had not been given the MMR vaccination - developed measles.
He is from south London, but it is not yet clear if he is one of the confirmed cases.
Joel is a little boy with measles.
His father admitted he had not been given the measles, mumps and rubella vaccination.
He said they had spoken to the doctor, the nurse and the health visitor and had still not felt confident enough to go ahead.
"We didn't know what to do for the best."
He added: "I wish we could have been more reliably informed at the time. I don't feel guilty about it.
"At the time, had we given him the MMR we would have felt as unsettled in ourselves".
Breathing problems
Initially it was thought Joel simply had high temperature and felt a little unwell.
His father said: "It wasn't really confirmed as measles until five days later when we took him to the GP."
"I was afraid because I'm aware it is a very serious disease and could have serious consequences.
"We watched him very closely and hoped that he would pick up."
Joel is now much better and his father says he is "on the mend".
Breathing problems which he experienced at the height of his illness have gone, he said.
He said he could have picked measles up anywhere - at school, where at least two other children have or have had measles, at the swimming pool, or at any of the children's clubs he goes to.
The couple have a younger son, Miles, and Joel's illness has made them reconsider their decision.
Their father said: "If Miles does pick up the infection then he would have a natural immunity.
"But if he doesn't then we have decide we will go ahead with the MMR for him - if in the next two weeks he hasn't developed it.
"It's still a difficult decision because I still think there could be a risk. It's a big injection."