BALB/c mice were infected with either RSV recWT or RSV M-null and monitored daily for (A) weight loss and respiratory parameters including (B) Penh, (C) EF50, and (D) respiratory rate by whole body plethysmography. Data are presented as mean ± SEM of 6 independent experiments (n=35 for recWT and n=36 for M-null). Groups were compared using Student’s t test, * p<0.05, ** p<0.01, *** p<0.001.