A culture of poor care is like a cancer - it will keep on spreading if not stopped, the barrister who chaired the public inquiry into the Stafford Hospital scandal has warned.
Robert Francis QC said bad practice starts in "small places" but is passed on to "bigger places" unless something is done.
Despite his damning report three months ago, he remained proud of the NHS.
He said it was the country's "most valuable social asset".
Mr Francis, who was speaking at a conference in London organised by the patient group Action Against Medical Accidents, refused to say whether he was happy with the government's response to his report.
Ministers have stopped short of agreeing to recommendations such as a system of registration for healthcare assistants and making it a criminal offence to lie to regulators.
They have also proposed making nurses work for a year as healthcare assistants at the start of their careers - an idea that the Royal College of Nursing has branded stupid - even though it was not put forward by the Francis inquiry.
Mr Francis said he did not want to get into discussing the merits of the government's response.
But he was adamant that without action bad practice would spread.
"There is always a challenge with a vast organisation that the culture will slip in little places and then bigger places. It is like a cancer."
His £13m public inquiry into why the scandal at Stafford Hospital - where hundreds of needless deaths were caused by abuse and neglect in 2005-08 - was not picked up earlier.
It concluded that patients had been "betrayed" because the NHS had put corporate self-interest ahead of patients.