Such transfers create virulent strains from previously avirulent strains. Such transfer creates virulent strains from previously avirulent "Agrobacteria". RHDV appears to have evolved from a pre-existing avirulent rabbit calicivirus. Many species in this genus can be characterized as opportunistic avirulent plant symbionts. Furthermore, mutations in P1 or degradation by trypsin treatment yield avirulent "M. pneumoniae" cells. Mouse models have shown that the unencapsulated forms are avirulent. The avirulent strain, CMM100, does not contain these plasmids. Most environmental isolates are avirulent; however, isolates recovered from porcine sources contain human pathogenic serogroups.