An outbreak of measles has struck up to 21 children across South Yorkshire, it has emerged.
The immunisation was introduced in the UK in 1988
The Health Protection Agency said five cases had been officially confirmed and another 16 children were showing "extremely convincing" symptoms.
All those affected are children aged between just three months and 12 years who have not had the MMR inoculation.
Eleven of the cases are in Sheffield, the first outbreak of the disease in the city for five years.
The others are in Barnsley and Rotherham.
Letters have been sent to family doctors to alert them to the outbreak.
Dr Rosy McNaught, from the South Yorkshire Health Protection Agency, urged parents to let their children have the measles, mumps and rubella vaccine.
She said: "It is the only way to prevent measles. Measles is the most infectious childhood disease.
"It cherry-picks the children who have not been immunised.
"These children who have been getting measles have been horribly ill with it.
"It's only when your child gets measles that you can understand what a horrible illness it is."
Dr McNaught said she expected more cases to come to light over the next few days, as several of the children seemed to have contracted measles during the last week of the school term.
She said children with weakened immune systems, such as those with cancer or leukaemia, could die if they come into contact with the disease.
The MMR vaccine was introduced in the UK in 1988 and by 1992 more than 90% of children were being immunised.
Children are given a first dose at 12 to 15 months and a second, booster dose, at between three and five years old.
Take-up of the vaccine has dropped in recent years because of research claims it may be linked to autism.
But the research, first published in 1998, has since been widely discredited.