Abstract: A method for preventing and treating parasitic infections in animals by administering a lysine analog, such as EACA, to the animals on a continuous basis. In the preferred embodiment, the method of the present invention is directed to preventing and treating coccidial infections in poultry by adding EACA to the daily diet of a poultry flock. EACA may also be administered in ovo before hatching. The administration of EACA enhances the natural immune response of the poultry to the invading coccidial organisms and enables the poultry to combat the parasites without the need for antibiotics. Another aspect of the present invention involves preventing parasitic diseases in humans and animals by prophylactically administering a serine protease inhibitor, such as EACA, as an adjuvant in conjunction with a conventional vaccine effective against the target parasite.