Humans exist in a world fraught with exposure to microbial threats. Even baker's yeast has begun displaying virulence in an increasing number of cases. Our bodies employ a vast arsenal of antimicrobial defenses to ward off infection. Indeed, there is an ever expanding body of knowledge concerning the defenses mounted against pathogens. One potent antimicrobial defense involves the generation of radical species. These highly reactive molecules are capable of damaging nearly every cellular component of the microbe. Despite the significant threat posed by pathogenic organisms, correspondingly little is known about how microbes evade and resist elimination by the host. Even the contribution of antioxidant responses to virulence has only been studied minimally. Furthermore, nonpathogenic organisms also often possess defenses against oxidants that are similar to defenses found in pathogens. How is it, then, that the antioxidant defenses of virulent organisms help them more successfully evade oxidant-producing immune cells? To address this topic, a physiologically relevant macrophage co-culture system has been developed and applied to study oxidative stress responses in the typically avirulent baker's yeast, Saccharomyces cerevisiae, and in its pathogenic relative Candida albicans . The production of reactive nitrogen intermediates by macrophages appears to factor heavily in the killing of each of these yeast. Comparative studies of the nitric oxide (NO) defenses employed by S. cerevisiae and C. albicans reveal that both yeast possess flavohemoglobin-dependent mechanisms for NO consumption. However, C. albicans NO defenses are highly inducible and appear to be NO specific, setting them apart from similar defenses in Saccharomyces and various bacteria. Furthermore, C. albicans flavohemoglobin expression is induced during exposure to NO-producing macrophages. There is no flavohemoglobin homologue in the human genome, which may point to the Candida albicans flavohemoglobin-dependent NO defense system as a potential chemotherapeutic target for the development of new antifungal technologies. Although much remains to be learned about flavohemoglobin-based defenses, it seems likely that the unique NO consumptive characteristics of C. albicans contributes to its virulent status.