Chatting with practice sister Julie Lees at the Range Medical Centre, Mr Cameron said he should "set a good example" to the public by having a flu jab but would not "because I've got my speech tomorrow".
"Don't take my blood pressure today either. I think that would probably be off the chart with the worry," he joked.
Forcing benefits claimants to spend 35 hours a week applying for a job will "end the something for nothing culture", Iain Duncan Smith said today.
Mr Duncan Smith told the Conservative Party conference that the pilot scheme, which will make the long-term unemployed who cannot find work attend a job centre, would be targeted at those who looked as if they are about to drop out of the Work Programme.
It will ensure the long-term unemployed visit mandatory attendance centres at their local job centre to apply for interviews from 9am to 5pm every week day, the Work and Pensions Secretary said.
"Alongside what we have already done with mandatory work programme and our tougher sanctions regime, this marks the end of the something for nothing culture," he added.
A "parallel universe" with a Labour-Lib Dem coalition in power would mean three-day electricity blackouts, votes for prisoners and soaring taxes, Communities Secretary Eric Pickles has said.
Imagining what could have happened had the Tories not become the senior coalition partners, Mr Pickles warned of a "dour" Gordon Brown as Prime Minister, cursing Tony Blair and listening to the advice of former spin doctor Damian McBride and "policy wonk" Ed Miliband.
He told the party conference that the Cabinet would have featured disgraced ex-Lib Dem minister Chris Huhne - elevated to "Lord Huhne of Wormwood Scrubs" - championing prisoner voting rights and "Baron" Len McCluskey, with the Unite leader abolishing Margaret Thatcher's trade union reforms.
"In reality, Gordon may be absent. But they are the same old Labour Party," Mr Pickles added.
Boris Johnson reminded Tory conference delegates of his prediction that last year's London Olympics and Paralympics would spur a baby boom.
He told the Conservative Party conference: "I prophesied that the athletes had moved the people of this country to such paroxysms of excitement on the sofas of Britain that they had not only inspired a generation, but probably helped to create one as well."
"Like all my predictions and promises, I have delivered, in that GLA demographics say live births in London will be 136,942, which is more than in any year since 1966 when England won the World Cup."