Yes, it is true, and no, you did not miss anything major.
This review of Season 3 of "24" on Epinions.com covers your question.
"When we had left Season Two of 24, President Palmer had been infected with
some sort of disease and we see him falling to the ground in anguish. Season
Three actually doesn't pick up where that left off. Palmer is now fine and has
This review on the UK based DVD Times website indicates that a bit of time has
passed between Season 2 and Season 3, so the incident doesn't appear to be all
that topical, at least within the series.
"Its three years since President Palmer (Dennis Haysbert) collapsed following
his exposure to a deadly biological agent, his recovery has been slow but he
remains in power ready to face the biggest challenge of his Presidency."
Another review on this same site indicates that the attack has taken at least
slight toll on the President Palmer.
"Looking well three years on from his assassination attempt, Palmer seeks to
stay in office for another four years. He?s also prepared to fight back against
those who believe he is unfit to govern as the president suffers from the odd
loss of breath now and then - a side-effect from his near fatal attack at the
end of day two."
So, it looks like the passage of time, and more pressing business, has made the
attack on President Palmer "old news" by the time that Season 3 starts.
If you need any clarification, please feel free to ask.
Google search on: 24 "President Palmer" disease
Google search on: 24 "Season Two" "Season Three" Palmer
Looking Forward, denco-ga - Google Answers Researcher