There are now 25 cases of measles confirmed in the midwest region since the beginning of the year - half of those cases have been hospitalised.

The majority are in Limerick city - one is in Clare but linked to Limerick.

Almost two-thirds of the cases are aged between 15 and 40 which health officials in the area say indicates that measles is not just an illness of childhood.

In a statement, the Mid West Community Health team said the illness is a serious public health issue and can spread rapidly among people who are not immune.

If one person has it, 90% of the people close to that person who are not immune also become infected.

It is spread through coughing and sneezing.

It can lead to serious complications particularly in young children, such as pneumonia and inflammation of the brain, and can, in rare cases, lead to a fatal disease of the brain which develops years after the infection.

The current outbreak is spreading they believe, because the proportion of the population in the region who are immune to the illness is less that 95%.

The only effective protection against measles is two doses of the MMR vaccine, and the Health Service Executive is providing free clinics in Limerick city tomorrow and each Thursday for the next three weeks at its Barrack View Primary Care Centre on Edward Street.

As part of the outbreak control measures, the vaccine is also being provided free through GPs.

The HSE is also warning adult groups aged under 40 who have not been vaccinated to do so, particularly parents, those who work with the public, women planning to get pregnant (as the MMR cannot be administered during pregnancy), and those planning to travel to Europe, as there are a number of large outbreaks of the illness there - in the UK, France, Italy Germany and Romania.

Measles can be easily transmitted in the confined spaces of an aircraft to people who are immune.