That's according to a TU Dublin study.
One in five census areas detected infection cases caused by cryptosporidium over 10 years.
The Irish Times reports just over 3,400 out of 18,600 localities saw illnesses due to the common water-based parasite.
The data's included in a nationwide analysis into the bug, which causes gasto-intestinal issues.
Symptoms include weight loss, vomiting, diarrhoea, abdominal pain, nausea and fever.
Study reveals extent of challenges posed by common waterborne bug https://t.co/ijaIQOotNJ
— The Irish Times (@IrishTimes) September 3, 2021

Worst Prison Overcrowding In History Of State, According To Prison Officers Association
Over 12,000 Defects Remain On New National Children's Hospital project.
Minister Martin Heydon Speaks As Fuel Scheme Announced
Sallins Man Part Of Intercepted Global Sumud Flotilla
One In Seven Households Unable To Afford Adequate Home Heating
Fifth Arrest Made In Connection With Fatal Edenderry Fire
Two People, Including A Teenage Boy, Arrested After Gun Incident In Ballymun
Man Charged With Fatal Assault Of Woman In Waterford