Unusual strains of the pathogen Candida albicans have been found that contain a single set of chromosomes. Formation of such haploid strains weeds out damaged copies of genes to promote evolution in the human body. See Article p.55 The common human fungal pathogen Candida albicans has long been considered an obligate diploid organism, with a rare, parasexual tetraploid stage and no meiosis. This has hindered classical genetic studies and made molecular manipulations more difficult than in model yeasts like Saccharomyces cerevisiae. Now Judith Berman and colleagues have identified a viable haploid C. albicans state derived from diploid cells. These cells can be isolated from in vitro stress conditions or following in vivo passage through a mammalian host. Haploids and their auto-diploids are significantly less fit in vivo when compared to heterozygous diploids. The authors have constructed a number of stable haploid strains to facilitate molecular and genetic analyses of C. albicans biology and virulence.