More than 700 Dubliners have been struck down with mumps so far this year.
New figures from the HSE show the majority are aged between 15 and 24-years-old.
The viral infection causes fever, headaches, swollen glands and just a general feeling of discomfort and is passed from person to person.
So far this year, there have been 1,793 cases reported across Ireland.
It can be prevented by the routine MMR vaccine given to children.
Adults can avail of the free vaccine with the only charge being their GP administration charge.
Children are routinely vaccinated at 12 months, with a booster in Junior Infants.
According to the HSE’s assistant national director for public health, Dr Kevin Kelleher, immunisation rates in Ireland have dropped to 91%, and are as low as 80% in some parts of the country.
While vaccination rates are down, control of both mumps and measles is very good, he added.
Both are very serious diseases which can have dramatic outcomes such as encephalitis and meningitis along with infertility for men.
It is worrying that between 6,000 and 7,000 children every year are not vaccinated, said Dr Kelleher.
Not alone are they putting others at risk, but they are also suppressing their own immune systems.
"These are the most infectious diseases, one infected person can infect 10 to 20 people in a room. On a bus they could infect 15 people."
If Ireland can get its vaccination rate up to 95% both diseases would be a feature of the past, he said.
Mumps causes swollen neck glands and a fever.
Anyone with such symptoms is advised to stay at home so they do not infect other people.