while considerable advances in water supply and sanitation have occurred in oecd countries pathogens can still enter water supply systems due to resistance to disinfection eg giardia cryptosporidium and enteric viruses treatment system deficiencies eg inadequate disinfection periodic treatment failures or distribution system contamination excessive rainfall and agriculture runoff was the cause of the contamination in europe significant outbreaks of waterborne pathogenic organisms have occurred in sweden widerstrom et al