Mr Ostrowski met Chancellor Alistair Darling on an earlier visit
Labour's candidate in the Norwich North by-election has been treated in hospital for suspected swine flu.
A scheduled visit by Lord Mandelson was cancelled because party workers had been in contact with Chris Ostrowski.
It is not known if he has swine flu but a party spokesman said "every possible precaution" was being taken.
Twelve people are standing in Thursday's by-election. Labour MP Ian Gibson resigned after he was deselected over his expenses claims.
A spokesman for Labour's Norwich North Labour campaign said Mr Ostrowski had felt unwell and was taken to hospital on Tuesday morning.
He said he had collapsed at home and was taken to hospital by ambulance. He was released from hospital on Tuesday afternoon.
"Clearly, our priority is to ensure that nobody falls ill from swine flu unnecessarily and we will follow the medical advice and take every precaution necessary," the spokesman said.
Collapsed at home
Dr Gibson had a majority of 5,459 over the Conservatives in 2005 but announced he was quitting on 5 June amid a row about the way he had been treated by the party.
The by-election is the first parliamentary contest since the MPs' expenses scandal broke and the parties have been campaigning hard in the constituency.
The Conservatives have been running a high profile campaign for their candidate Chloe Smith - party leader David Cameron made his fifth visit to the constituency on Monday. April Pond is standing for the Liberal Democrats.
Rupert Read is standing for the Greens, Glenn Tingle for the UK Independence Party and Robert West for the BNP.
Among the independents standing is Craig Murray, former Ambassador to Uzbekistan, who left the post in 2004 after criticising its human rights.
Laud Howling is standing on behalf of The Official Monster Raving Loony Party while Thomas Burridge is representing The Libertarian Party and Anne Fryatt is representing the None of The Above Party.
Peter Baggs and Bill Holden are also standing as independents.