The genetic stability of every living organism depends on accurate DNA replication and repair systems. Here, we investigated the mismatch repair (MMR) gene MshA and how it impacts virulence and the evolution of azole resistance. We examined gene variation in 62 environmental and clinical strains. We have observed 12 strains with variants (18.2%), and 8 strains among them showed missense variants. We demonstrated that null mutants are haploid and have conserved karyotypes with discrete gross chromosomal rearrangements. The Δ strains are not sensitive to several DNA-damaging agents. The lack of caused a significant reduction of virulence of in a neutropenic murine model of invasive pulmonary aspergillosis and in the invertebrate alternative model Wild-type and Δ populations did not show any significant changes in drug resistance acquisition after they were transferred 10 times in minimal medium in the absence of any stress. However, these populations rapidly acquired virulence in the Δ background and high levels of resistance to posaconazole in the presence of this drug (at least 200-fold-higher levels of resistance than those derived from the wild-type strain). Taken together, these results suggest that genetic instability caused by Δ mutations can confer an adaptive advantage, mainly increasing posaconazole resistance and virulence acquisition. Invasive aspergillosis (IA) has emerged as one of the most common life-threatening fungal diseases in immunocompromised patients, with mortality rates as high as 90%. Systemic fungal infections such as IA are usually treated with triazoles; however, epidemiological research has shown that the prevalence of azole-resistant isolates has increased significantly over the last decade. There is very little information about the importance of genomic stability for population structure, azole resistance, and virulence. Here, we decided to investigate whether the mismatch repair system could influence azole resistance and virulence, focusing on one of the components of this system, Although the mutation frequency of (the homologue) is low in environmental and clinical isolates, our results indicate that loss of function can provide increased azole resistance and virulence when selected for. These results demonstrate the importance of genetic instability in as a possible mechanism of evolving azole resistance and establishing fitness in the host.