Michael Howard accused Tony Blair of failing to tackle hospital superbug MRSA, during a commons clash which saw Mr Blair forget what he had been asked.
Mr Blair lost his thread at question time
A red-faced prime minister had to ask Labour MPs to jog his memory.
Mr Howard said hospital cleanliness was an "important issue" and Mr Blair's memory slip was down to the PM not answering his questions properly.
Mr Blair agreed more needed to be done about MRSA but he said Tory calls to cut targets were not the answer.
'More to do'
The prime minister began his reply to Mr Howard's question by defending the government's decision to increase national insurance to pay for NHS improvements.
But he halted in mid-flow and admitted the second part of Mr Howard's question had "now just slipped my mind".
With MPs on all sides in uproar, the prime minister turned to Labour MPs to ask what the question was before regaining his composure.
"It is a major issue, we accept that. That is why we are investing round about £300m extra in hospital cleanliness over the next couple of years."
He said the government "still had more to do" on the issue, but cutting investment, which he claimed the Tories would do, was not the answer.
Mr Howard, who had ealier criticised Mr Blair for dodging difficult questions, replied: "The problem of hospital cleanliness is rather an important one and if the prime minister concentrated on answering the questions I am putting to him it wouldn't have slipped his mind,"
He said the number of deaths from the MRSA superbug had doubled under Labour and NHS managers, including the organisation's chief executive, believed Labour's performance targets "make matters worse".