It'd be absolutely terrifying - a totally invisible threat setting off an alarm that's telling you there's a danger in your immediate vicinity. And then what?

It'd be absolutely terrifying - a totally invisible threat setting off an alarm that's telling you there's a danger in your immediate vicinity. And then what? The office building fire drill, where hundreds of people all trudge down 20 stories together in the claustrophobic confines of a concrete stairwell along with the unconsciously contagious? Or lock down the room until someone can decontaminate the whole thing?