This one is up to you! The rules are mum about it. In a standard superhero game [four color, at least] I would rule that guns and other attacks don't automatically do damage that is somehow more lethal than Doctor Doom's blaster. Some games, however, may require a more lethal tone. In another forum I said the following regarding grittier-and-grimmer games:
I recommend ruling that characters who lose twice their value in a Resistance are either dead or permanently disfigured, depending on the narrative requirements and the Resistance being affected.
I also recommend that characters who go into negative numbers (without reaching twice their value) heal at a slower rate.
Example) Thrasher has Fortitude 3d. He takes 2d worth of Fortitude damage in combat. Tha heals normally, so he's up at full for the next fight. In that fight, however, he suffers 4d worth of Fortitude, placing him at -1. He is incapacitated, and the Narrator rules that it will take him longer than normal to heal that damage. [Days? Weeks? I still haven't decided.] A week later, Thrasher is at Fortitude 2d. He suffers 5d of damage, which puts him at -3d. That's twice his maximum value of 3d! Poor Thrasher is D-E-A-D!