Fortitude defense is based on Stamina and measures health and resistance to threats like poison or disease. It incorporates constitution, ruggedness, metabolism, and immunity.
RESISTANCE CHECKS[]
Defenses are also used to measure the ability to overcome certain effects, involving a resistance check of the defense plus a die roll against a difficulty class determined by the effect or hazard. So you might make a Fortitude resistance check for your hero to overcome a toxin, for example, or a Dodge resistance check to avoid a trap just as it is triggered, and so on. This is referred to as “resisting,” such as “resisted by Fortitude” or “resisted by Dodge”.
References[]
Mutants & Masterminds Hero’s Handbook, Copyright 2011, Green Ronin Publishing; Author Steve Kenson. Chapter-3 Abilities, Pg 54-59
D20Hero SRD Character Creation Article-http://www.d20herosrd.com/character-creation