The Zalkonians had a weapon which could apparently choke/suffocate everyone on the Enterprise-D, it is unknown how it worked (by effecting the nervous system that controls breathing, etc) no warning was issued by the computer that life support was failing. It is unknown if Data was effected, however. He is seen rising from the floor, but he may have been helping someone who was effected.
A lot of "unknowns" in this. Placing it here in case someone wants to take a stab at something a little less vague and more about the actual weapon. - Archduk3