Most of the histopathological features of influenza viral pneumonia listed above are also seen in mice, but some signs and symptoms of human influenza are rarely observed. Mice do not show outward signs of fever and do not have increased rectal temperatures, nor is dyspnea, cyanosis or hemoptysis easily, if ever detected in mice infected with strains other than the H5N1 virus. However, as in humans, reduced blood oxygen saturation levels, a measure of lung function, can be measured in mice, and these levels are dramatically lower as pneumonia progresses and the mice approach death (Barnard et al., 2007). Weight loss is also a good marker of disease severity (Sidwell and Smee, 2000). The reader is referred to the following recent papers that show the utility of the seasonal influenza mouse model for evaluating vaccines (reviewed by van der Laan et al., 2008, Hagenaars et al., 2008, Hai et al., 2008) and antiviral drugs (Dimmock et al., 2008, Gilbert and McLeay, 2008, Reading et al., 2008, Smee et al., 2008, Wang et al., 2008).