As shown in Table 1 , virus-infected mice showed a slightly decreased partial pressure of arterial oxygen (Pa O2 ), saturation of arterial oxygen (Sa O2 ), and slightly increased partial pressure of arterial carbon dioxide (Pa CO2 ) from days 4 to 6 p.i. Most infected mice presented with severe clinical signs of respiratory distress on day 8 p.i., and blood gas analysis also showed that Pa O2 and Sa O2 dramatically decreased compared with the controls (p,0.05). These results suggested acute respiratory dysfunction and severe hypoxemia in virus-infected mice.