Pathogenesis STD adds the fourth tract to the body, the Genitourinary tract, introducing pathogens, traits, and virulence factors appropriate for this area of the body. Further, keeping to the science, the Genitourinary tract has no toxin cards but instead a new mechanic has been introduced, Collateral Damage cards! Collateral Damage cards add the damage from the immune system's attack against your pathogens to the damage your pathogens deal to the body if your pathogens survive the immune system's attack. A copy of the Pathogenesis base game is required to use this expansion.