Umar Yar'Adua is still in the race for the presidency, the People's Democratic Party candidate told the BBC today.
He spoke from his German hospital bed to quash rumours he had died.
Yar'Adua is suffering from a "small infection", President Olusegun Obasanjo told a campaign rally in Abeokuta.
The candidate denied newspaper reports he collapsed in Lagos airport. Party chieftain Olabode George told journalists Yar'Adua began to have breathing difficulties in a meeting on Tuesday night.
He was taken to a private clinic in Abuja and then flown to Germany on the advice of a doctor.
PDP party spokesman John Odey said: "At no time was he incapacitated."
The Nigerian Sun newspaper's headline this morning was "Deadly Ill" over a picture of Yar'Adua. The Times of Nigeria, a web-based news service, published reports that the candidate died this morning.
But there are some in the PDP who feel the candidate must be replaced.
Tomorrow the Independent National Electoral Commission will publish the final list of candidates who will stand for election. The deadline for substitutions was in February.
The Electoral Act gives provision for postponing elections in the case of the death of a candidate, or as a result of a "natural or man made disaster."
Many people believe the elections won't go ahead. Last year the president staged a bid to extend his tenure of office, and many suspect the existence of a plot to install an Interim National Government, under the control of the president.