The ability of to cause disease in humans varies significantly among strains with highly related genotypes. In general, environmental isolates of pathogenic species such as var. have reduced virulence relative to clinical isolates, despite having no differences in the expression of the canonical virulence traits (high-temperature growth, melanization, and capsule formation). In this observation, we report that environmental isolates of tolerate host CO concentrations poorly compared to clinical isolates and that CO tolerance correlates well with the ability of the isolates to cause disease in mammals. Initial experiments also suggest that CO tolerance is particularly important for dissemination of from the lung to the brain. Furthermore, CO concentrations affect the susceptibility of both clinical and environmental isolates to the azole class of antifungal drugs, suggesting that antifungal testing in the presence of CO may improve the correlation between azole activity and patient outcome. A number of studies comparing either patient outcomes or model system virulence across large collections of isolates have found significant heterogeneity in virulence even among strains with highly related genotypes. Because this heterogeneity cannot be explained by variations in the three well-characterized virulence traits (growth at host body temperature, melanization, and polysaccharide capsule formation), it has been widely proposed that additional virulence traits must exist. The natural niche of is in the environment, where the carbon dioxide concentration is very low (∼0.04%); in contrast, mammalian host tissue carbon dioxide concentrations are 125-fold higher (5%). We have found that the ability to grow in the presence of 5% carbon dioxide distinguishes low-virulence strains from high-virulence strains, even those with a similar genotype. Our findings suggest that carbon dioxide tolerance is a previously unrecognized virulence trait for .