Treatment of systemic fungal infections is difficult because of the limited number of antimycotic drugs available. Thus, there is an immediate need for simple and innovative systems to assay the contribution of individual genes to fungal pathogenesis. We have developed a pathogenesis assay using Caenorhabditis elegans, an established model host, with Saccharomyces cerevisiae as the invading fungus. We have found that yeast infects nematodes, causing disease and death. Our data indicate that the host produces reactive oxygen species (ROS) in response to fungal infection. Yeast mutants sod1⌬ and yap1⌬, which cannot withstand ROS, fail to cause disease, except in bli-3 worms, which carry a mutation in a dual oxidase gene. Chemical inhibition of the NADPH oxidase activity abolishes ROS production in worms exposed to yeast. This pathogenesis assay is useful for conducting systematic, whole-genome screens to identify fungal virulence factors as alternative targets for drug development and exploration of host responses to fungal infections.Nosocomial microbial infections are a growing health problem. Among these, fungal infections are especially threatening, with an estimated mortality rate of 40% (47). The key reason for this alarming mortality rate is the limited range of antifungal agents. Identification of new drug targets requires highthroughput infection assays that are complicated by the very fact that they involve two organisms: a host and a pathogen.We have taken a reductionistic approach to studying hostpathogen interactions and have developed a Saccharomyces cerevisiae-based assay to understand the genetic and molecular mechanisms of fungal pathogenesis. Using Caenorhabditis elegans as a model host, we have found that S. cerevisiae infects the worm, producing visible disease phenotypes. The two organisms used in our study are specifically suited for host-pathogen infection studies because both genomic sequences have been completely determined and mutants are readily available. A complete genome knockout collection is available for S. cerevisiae, a resource that does not exist for any fungal pathogen. Likewise an RNA interference (RNAi)-mediated knockdown genomic library is available for C. elegans. These unique tools are key in the context of a genetic screen and allow us to systematically scan the entire genomes to identify fungal virulence factors and modulators of host immunity that combat a fungal pathogen.The budding yeast S. cerevisiae has recently been described as an emerging pathogen and has been isolated from human patients (34,35). It is routinely used as a model for pathogenic fungi because a large proportion of its genes are conserved in pathogenic fungi (for a review, see reference 32). Homologs of genes and pathways identified in S. cerevisiae have been shown to be important in bona fide pathogens. It has also been used for the identification of gene products important for fungal survival in the mammalian host environment (21, 46). For example, the SSD1 allele type affects pathogenicity of yea...