A disposable active humidifier for the mechanical ventilation of a patient, which has the particularity that it comprises a cartridge which forms a humidification chamber which is delimited by an inlet and by an outlet and can be interposed in the ventilation circuit. The cartridge has an interspace which is externally delimited by a heat exchange surface and is internally delimited by a hydrophobic membrane which surrounds the humidification chamber. A humidification fluid, originating from a bottle or bag, can be introduced in the interspace.