Mammals are treated for infections or for conditions associated with pathologically proliferating mammalian cell growth by administration of a manipulator of nitrosative stress to selectively kill or reduce the growth of the microbes or helminths causing the infection or of host cells infected with the microbes or of the pathologically proliferating mammalian cells. Novel agents include -alkyl-S-alkyl-homocysteine sulfoximines wherein the -alkyl contains 2 to 8 carbon atoms, and the S-alkyl contains 1 to 10 carbon atoms. Mammals in need of increased nitrosative stress defenses are treated, e.g., humans at risk for a stroke because of having had a transient ischemic attack, are treated. Treatments to increase nitrosative stress defenses include repeated administrations of low doses of manipullators of nitrosative stress so that subject treated has increased tolerance to nitrosative stress. Mammals are treated for protozoal infections by systemic administration of L-buthionine-S-sulfoximine and agent that increases nirtrosative stress.