Immunity against protozoan is conferred on an animal by a method comprising orally administering to an animal a transformed plant cell comprising a polynucleotide encoding a protective antigen against protozoiasis development, a transformed plant or its progeny or clone comprising the transformed cell, a propagation material of the plant or its progeny or clone, a processed material or extract of the above cell, plant, or its progeny or clone, or propagation material, or a protective antigen against protozoiasis development isolated from the transformed plant cell or the transformed plant or its progeny or clone.
