Six-week-old female mice were infected intranasally (i.n.) with 10 2.5 pfu SD/09 virus. Some of the infected mice showed signs of illness, such as altered gait, inactivity, ruffled fur, and anorexia on day 2 post-infection (p.i.). From day 2 p.i., the body weight of most mice significantly decreased ( Figure S1 ). By day 6 p.i., most mice presented with more severe clinical signs of respiratory disease, including labored respiration and respiratory distress, and most mice lost almost 20% of their initial body weight. On day 8 p.i., most mice were nearly unable to respond to exterior stimuli, and acute respiratory rates and labored respiration were observed (Video S1, and Video S2 for control). Approximately 60% of mice died between days 8 and 10 p.i. Gross observation of infected mice showed that the lungs were highly edematous, with profuse areas of hemorrhage and consolidation. No obvious gross lesions were observed in the kidneys, liver, spleen or brain of infected mice.