Several parasites responsible for mammalian diseases are dependent on cysteine protease for various life-cycle functions. Inhibition or decreasing function of these proteases can be useful in the treatment and/or prevention of these parasitic diseases including; toxoplasmosis, malaria, African trypanosomiasis, Chagas disease, leishmaniasis, schistosomiasis, amebiasis, giardiasis, clonorchiasis, opisthorchiasis, paragonimiasis, fasciolopsiasis, lymphatic filariasis, onchocerciasis, dracunculiasis, ascariasis, trichuriasis, stronglyoidiasis, trichostrongyliasis, trichomoniasis or cestodiasis.