City of Lincoln Council have revealed the levels of legionella bacteria found at a sheltered housing block in Lincoln were almost 12 times the safe level.
The council also says the water system is only checked every six months. They're now chemically flushing out the system, and say it's entirely a precautionary measure.
Legionella can cause Legionnaire's disease if it becomes airborne and water particles are inhaled.
Elderly residents have been told not to turn their taps on and have been provided with bottled water and off site washing facilities.