NHS Lanarkshire said routine water sampling had discovered legionella bacteria in the renal and endoscopy units at Monklands Hospital in Airdrie.
Filters have now been placed on basins and shower outlets in the units and in a ward served by the same water tank.
No patients are showing any signs of the disease.
Susan Friel, nurse director for acute services at NHS Lanarkshire, said: “We are working closely with microbiology and facilities colleagues to put in place further steps following these results to ensure the safety of patients and staff.
“This includes sampling on a regular basis until we have a full set of negative samples, and filters remaining in place for as long as required.
“We want to reassure our patients and staff that the risk of contracting legionella disease with this particular strain is extremely low and the measures we have taken are precautionary while we continue to sample the water.”
She added that infection prevention and control measures are in place and no patients are showing signs of legionnaires disease, but staff will continue to monitor the situation in the coming days.
Christina Coulombe, head of infection prevention and control at the health board, added: “There have been no toilets closed but handwashing facilities are out of commission as a precautionary measure while testing is ongoing.
“The option of portable sinks was discussed with staff and have now been provided in the areas requested by staff. Senior staff in all three areas can request further portable sinks by contacting facilities colleagues.”