Record numbers of young people North and South have been treated for mumps so far this year.
In Northern Ireland almost 1,600 cases have been reported to the Department of Health, while in the Republic 111 cases were reported to the Health Protection Surveillance Centre (HPSC) in the first six weeks of the year. This is 22 times the number of cases reported in the Republic during the first six months of last year.
Teenagers and young adults make up most cases, and doctors in the university area of Belfast are so concerned they are running special vaccination clinics for students.
A Department of Health spokesman in Northern Ireland said the 1,588 cases recorded since the start of the year was the highest number since mumps became a notifiable disease in 1990.
Two weeks ago the department said there had been 300 cases since the start of the year, and Northern Ireland's chief medical officer, Dr Henrietta Campbell, said it was significant that most of those affected had received only one dose of a mumps-countering vaccine.
She said two doses of vaccine such as MMR were needed to ensure protection.
Yesterday Dr Suzanne Cotter, a specialist in public health medicine with the HPSC in Dublin, said the cases were occurring mainly among students in the 18-24 age group.
The MMR vaccine was only introduced as part of the childhood vaccination programme in the Republic in 1988, and as a result many of these students would not have got the vaccine or would only have had one dose.
She emphasised that mumps, which is preventable through vaccination, could be a serious disease and could cause deafness, male sterility and encephalitis.
The main symptom reported during this outbreak, she added, was inflammation of the salivary glands.
- (Additional reporting PA)