The order to boil water has been lifted in Galway, but a senior public health expert said it was lucky that nobody died as a result of contracting the water borne cryptosporidiosis illness.
Director of Public Health for HSE West, Dr Diarmuid O'Donovan, said 40 people became seriously ill and were hospitalised during the outbreak. A small number of those are still at risk of ongoing long-term medical complications.
Dr O'Donovan also said that water supplies are now safer than they have ever been. Screening treatments are in place to remove any harmful organisms from water supplies.
Up to 90,000 people had to either boil water before use or buy bottled water for the past five months following an outbreak of cryptosporidium in March.
Officially about 240 people were confirmed as having contracted the severe waterborne stomach illness. However health experts say the real figure is probably more likely to be in the region of 5,000 or more.
HSE West said that the incidence of the disease has declined markedly and has now reached the levels normally seen at this time of the year.
They added that because of both this and the presence of new and effective water treatment systems, the directive to boil all water before use is being lifted.
In a statement the HSE said that the source or sources of the outbreak may never be identified but that detailed investigations into its cause will continue.
The impact of the outbreak on the city's tourist industry will not be fully known until the end of the year.