Green Strain: The most common of all the infected strains. If a survivor is bitten by a Green Strain, they have around an in-game week to find a green antidote before the survivor dies. Green Strain infected possess no special traits.
(First Example: TomCat)
Blue Strain: The slightly rarer strain. If a survivor is bitten by a Blue Strain, they have an in-game day to find a blue antidote before the survivor dies. Blue Strain infected have slightly more health than the Green Strain infected.
(First Example: El Dorado)
Red Strain: Much rarer than both Green and Blue. If a survivor is bitten by a Red Strain, they have a matter of minutes to find a red antidote before the survivor dies. Red Strains have more health than the Green or Blue Strains.
(First Example: Corpse Valley)
White Strain: The 2nd rarest strain. If a survivor is bitten by a White Strain, the survivor has approximately 3-5 seconds to live before they die. With no antigen available, it's essentially the kiss of death for the afflicted survivor. White Strains have far greater health than Red, Blue, or Green strains.
Gray Strain: The rarest of all strains. If a survivor is bitten by a Gray Strain, the survivor presumably gets knocked out, and they are infected with the Gray Strain. The map shows nothing under their names, which helps support the theory of it being the "Living Infected" Strain. Gray Strains has the same health as Green Strain, but there is no antigen for it.
(First Example: Devil's Dulley Gang)