Host age has also been identified as a factor in regulating the cross-species transmission of coronaviruses, Baric added. In mice infected with SARS-CoV, Baric noted that the virus will replicate efficiently but not cause disease. A mouse adapted virus—with mutations in the S and membrane (M) glycoproteins and the viral replicase resembling those that arose in humans during the 2003 SARS epidemic—may be created by passaging32 the virus multiple times through mice.