Health Secretary Shona Robison has apologised to patients and families affected by testing for bacteria in the water supply at two Glasgow hospitals.
Four children are now receiving treatment for infections which may be linked to the water supply at the Royal Hospital for Children (RHC).
Testing has also been extended to four wards at the neighbouring Queen Elizabeth University Hospital (QEUH).
Ms Robison told MSPs that "appropriate precautionary measures" were being taken by NHS Greater Glasgow and Clyde (NHSGGC) to protect patients.
The issue emerged on Friday but Scottish Labour MSP Anas Sarwar said parents had told him it had been ongoing for almost three weeks.
It has resulted in the inability of young cancer patients to bathe properly, with some taking taxis to other sites to use facilities, he said.
He added: "The news of contamination of the water supply in the cancer ward at the children's hospital in Glasgow has caused understandable worry and concern for parents of very sick children.
"Parents who tell me they learned more about the problem from a newspaper than any communication from the health board.
"They also tell me this issue has been running for three weeks but has only come into the public domain in the last few days."
Ms Robison said she had first been made aware of the matter on March 11, with Scottish Government officials informed prior to that.
"I absolutely understand the worry and concern of parents," she said.
"I have been assured by the health board that they have indeed been keeping parents informed.
"Of course I would apologise to the parents and the children for the inconvenience that they have experienced, but I am sure everyone would understand the most important thing here is safety, and if the shower heads and taps are being tested and investigated then that has to take its course.
"These are complex issues and they need to be fully, fully investigated, this bacteria is very, very rare indeed.
"Absolutely everything has been done to get to the bottom of this."
NHSGGC said it is working with experts from Health Protection Scotland, Health Facilities Scotland and Scottish Water.
A statement from the health board said: "Three children continue to receive treatments for infections that may be linked to bacteria found in the water supply.
"Tests are ongoing to confirm if they are indeed linked. A fourth patient has shown symptoms and has been readmitted to the RHC.
"There are no reports of any patients being infected by bacteria from water in the adult wards."
Infection control measures were introduced to affected RHC wards at the weekend and water filters are being installed.
The spokesman added: "As a result, it is hoped that the full water supply will return to normal within 48 hours after appropriate testing has been carried out and it is anticipated this will be mirrored at the QEUH.
"We would like to thank the patients and families in the wards affected in the RHC for their continuing patience and support while the testing was carried out and fitting of filters is being undertaken."