Currently, pyrimethamine is used for the treatment of parasite diseases including toxoplasmosis, actinomycosis, and isosporiasis, and for the treatment and prevention of Pneumocystis jirovecii pneumonia through the competitive inhibition of DHFR, thereby interfering with the regeneration of tetrahydrofolic acid from dihydrofolate [26]. This evidence concerns the gene DHFR and pneumocystosis.