It's one of the things they never bother with in games. Much like how in game, being shot in the weapon can hurt and kill you, regular humans can casually flip vehicles, Chief has 90 minutes of air in his armor, but can be in space for infinite amounts of time and dies immediately in bodies of water.
It's something that really only is considered in the lore of the books. In some of the animated stuff, even if your wearing armor so much as a swipe from them is infectious if it cuts to the skin. And.. they kind of have to be well rooted to do stuff like spread spores, like they only really start once they plastering the walls with flood goop, nor is it as immediately successful as just being assault by flood combat or infection forms.
Though, Samus MIGHT be immune to the flood. Metroids have the ability to consume an invisible "life energy" of what they prey one, This ability is lethal to anything NOT wearing a heavy duty shielded power suit in a matter of a couple seconds. This ability works on anything caught in it's Maw, or inside it's body. Samus inherited the ability in the latest game in the timeline, which rendered her immune to a similar parasite to the Flood, the X-parasite. She would use the energy absorbed from the X to restore her own armor's energy supplies. Bare in mind, purely speculative though, can't really be proven.