Higher than recommended levels of legionella bacteria were detected at St Stephens Pavilion
Residents at flats in Norwich said they are "furious" only 12 hours notice was given for them to leave their homes to clear them of the Legionella bug.
The bacteria which causes Legionnaires' disease has been found in two blocks of flats on the site of a former hospital.
The water tanks have been flushed out but on Wednesday night residents said they were asked to leave their flats by 0900 BST on Thursday for more cleaning.
Student Lauren Cook, who lives there, said:"It's been an absolute nightmare."
'Really inconvenient'
She is sitting a final examination for her degree at the University of East Anglia on Friday and said the 12 hours notice residents were given was not enough.
Ms Cook said: "It is really inconvenient and nobody will give us a straight answer as to what is going on.
"I have no idea when I can go home."
Legionnaires' disease facts
It is often contracted by inhaling mist containing the bacteria from water sources
It cannot be passed from one person to another
About 300 to 500 cases are reported each year in England
Clare Corbett, operations director of Residential Management Group (RMG) - which manages the homes - said: "As soon as we knew, we notified residents and environmental health was on site.
"We hand delivered notes to everyone and four members of staff have been on site ever since Monday."
Ms Corbett said 64 flats were affected and expert cleaners needed to turn on all the taps to flush through the system.
The firm apologised for any inconvenience or alarm the problem has caused and said the decontamination work would be finished by 1700 BST Thursday.
The apartments were built on part of the site of the old Norfolk and Norwich Hospital in St Stephens Road.
People living there have been told not to drink or bathe in the water.
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