Anglian Water has been criticised by a watchdog after a contaminated supply led to 250,000 people in having to boil their tap water in Northamptonshire.
In June this year, a rabbit got into a wash water tank, leading to the bug cryptosporidium infecting the firm's treatment works in Pitsford.
A warning was immediately issued to customers in Northampton and Daventry.
The Drinking Water Inspectorate said the water scare was caused by a failure of "basic water supply hygiene".
Cryptosporidium is a microscopic parasite can cause severe stomach upsets, especially in young or elderly people.
'Robust monitoring'
The Anglian Water oversight allowed a small rabbit to get into a wash water tank, contaminating the distribution system with a strain of the cryptosporidium parasite which infects the animals.
Deputy chief inspector Marcus Rink said : "My inspector is critical of this failure of basic water supply hygiene arrangements.
"However his assessment rightly highlights how it was Anglian Water's own robust monitoring procedures which raised the alarm and secured a quick and effective multi-agency response."
The announcement led to school closures and a shortage of bottled water in supermarkets across the area.
The inspectorate said 22 people were found to have contracted cryptosporidiosis due to the same strain of the parasite found in the rabbit and water samples.
Before this incident, only a single case of human infection with this strain had been recorded anywhere in the world.
The inspectorate has now commissioned studies into the rabbit genotype of cryptosporidium.
Chief inspector of drinking water Professor Jeni Colbourne said the incident showed how anything less than constant vigilance was unacceptable when dealing with water supply hygiene.
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