Idrissou powered home a header from Geremi's cross in the 86th minute to complete a second-half comeback from World Cup-bound Cameroon, who conceded an early goal and faced an embarrassing early exit from the tournament.
Cameroon, the 2008 runners-up, lost their opening match to Gabon and had they suffered a second successive defeat would have been the first side eliminated from the 16-team tournament.
Record-breaking defender Rigobert Song, who has played more Nations Cups than any other player, figured in two defensive slip-ups which allowed Zambia to score but Cameroon's power and strength pulled them through.
Jacob Mulenga took advantage of Song's botched attempted clearance to give Zambia an eighth-minute lead.
Cameroon made two halftime changes to launch their fightback but their equaliser came after a horrendous goalkeeping error from Kennedy Mweene who allowed Geremi's cross to slip under his body and into the net in the 68th minute.
Four minutes later, Samuel Eto'o extended his Nations Cup scoring record to 17 goals with a typically opportunist effort after an attempted clearance ricocheted off him and he was quickest to snap up the loose ball.
Zambia were awarded a penalty 10 minutes from time when goalkeeper Carlos Idriss Kameni brought down Mulenga, who had outstripped Song for pace.
Chris Katongo converted the kick but parity lasted for all of five minutes before Idrissou rose above the Zambia defence to head home another probing cross from veteran full back Geremi.
Cameroon moved up to second place in the group behind Gabon, who have four points after drawing 0-0 with Tunisia earlier on Sunday.
Tunisia are on two points and Zambia one but all four teams are still in the race for the quarter-finals, which will be decided on Thursday.
