Serial Passaging

Mice infected with MA15 virus die from an overwhelming viral infection with extensive, virally mediated destruction of pneumocytes, and ciliated epithelial cells.The MA15 virus has six coding mutations in its genome, which, when introduced into a recombinant SARS-CoV, confer lethality.. The MA15 virus will enhance the use of the mouse model for SARS because infection with this virus in mice reproduces many aspects of severe human disease, including morbidity, mortality, and pulmonary pathology.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1769406/