Daily and accumulated clinical score following aerosol challenge with either BRSV-Snk or BRSV-Dk. Six calves were experimentally infected with virulent BRSV, either passaged in vivo (BRSV-Snk, n = 3, calves A1-3), or in vitro (BRSV-Dk, n = 3, calves B1-3). Following infection on post-infection day (PID) 0, calves were monitored for seven days. Daily clinical scores (panel A) were calculated from observed clinical signs (see Tables 1 and 2). Accumulated clinical scores, from PID 0 to PID 7 (panel B), were calculated as the area under individual clinical score curves.