THE HSE is asking people to stay home from work or school if they are displaying symptoms of flu.
The official monitoring threshold was crossed last week as GPs across the country reported a sharp increase in flu cases.
Flu starts far more quickly than a cold, and symptoms include a high temperature, body aches, a dry cough and a headache.
Dr John Cuddihy said avoiding contact with people is the best way to stop the illness spreading.
He said: “Stay out of school or work or creche while they’re symptomatic until their symptoms have completely resolved, and that usually takes between five and seven days.
“People who develop the symptoms of flu, it’s important that they would cough or sneeze into a tissue, dispose of the tissue immediately in the bin, and wash their hands.”
“People in high-risk groups need to get vaccinated against flu if they have not done so already.”
The influenza-like illness rate for the first week of December was 37.5 cases per 100,000 population which is above the baseline threshold of 18.1 cases per 100,00, used to assess influenza activity.
“The HSE has a dedicated self-care website www.undertheweather.ie which provides tips and advice in relation to common winter illnesses,” said Dr Cuddihy.