A door (10, 10') for selectively blocking and unblocking a doorway through a wall, the door (10, 10') comprising: a first track; a second track spaced laterally from the first track to define a span between the first track and the second track; a curtain (14, 14') including a main section, a first lateral section, a second lateral section, and a leading section; the first lateral section extending into the first track, the second lateral section extending into the second track, the main section extending between the first lateral section and the second lateral section, the main section spaced apart from the first track and the second track, the leading section below the main section and extending between the first lateral section and the second lateral section, the leading section movable selectively to a closed position and an open position, the curtain (14, 14') to block the doorway when the leading section is at the closed position, the curtain (14, 14') to unblock the doorway when the leading section is at the open position, the curtain (14, 14') being in a normal state when the first lateral section in an area proximate the leading section is in guiding engagement with the first track, the curtain (14, 14') being in a breakaway state associated with an unfolded condition when the first lateral section in the area proximate the leading section is dislodged from the first track but not folded over onto the main section; and an unfolder disposed within the span between the first track and the second track; characterized by the curtain (14, 14') being in a breakaway state associated with a folded condition when the first lateral section in the area proximate the leading section is dislodged from the first track and folded over onto the main section to define an enfolded space between the first lateral section and the main section; the unfolder extending into the enfolded space as the leading section moves toward the open position while the curtain (14, 14') is in the folded condition; and the unfolder having a shape that is circular about an axis extending substantially parallel with the curtain (14, 14') when in the normal state. a first track; a second track spaced laterally from the first track to define a span between the first track and the second track; a curtain (14, 14') including a main section, a first lateral section, a second lateral section, and a leading section; the first lateral section extending into the first track, the second lateral section extending into the second track, the main section extending between the first lateral section and the second lateral section, the main section spaced apart from the first track and the second track, the leading section below the main section and extending between the first lateral section and the second lateral section, the leading section movable selectively to a closed position and an open position, the curtain (14, 14') to block the doorway when the leading section is at the closed position, the curtain (14, 14') to unblock the doorway when the leading section is at the open position, the curtain (14, 14') being in a normal state when the first lateral section in an area proximate the leading section is in guiding engagement with the first track, the curtain (14, 14') being in a breakaway state associated with an unfolded condition when the first lateral section in the area proximate the leading section is dislodged from the first track but not folded over onto the main section; and an unfolder disposed within the span between the first track and the second track; characterized by the curtain (14, 14') being in a breakaway state associated with a folded condition when the first lateral section in the area proximate the leading section is dislodged from the first track and folded over onto the main section to define an enfolded space between the first lateral section and the main section; the unfolder extending into the enfolded space as the leading section moves toward the open position while the curtain (14, 14') is in the folded condition; and the unfolder having a shape that is circular about an axis extending substantially parallel with the curtain (14, 14') when in the normal state.