generally first found in Buckeye in the Missing questline or in the El Dorado quest.
The Blue Strain will infect a human once bitten, and it will attack the host's brain until the victim dies, and the 'reanimation' process occurs after around 24 hours of infection. Symptoms include discolouring of skin, and eventually, death.
Once the Green Strain antigen was produced, a mutation took place, and the Blue Strain appeared. The virus ravaged towns and cities, crippling humanity. Eventually, scientists managed to manufacture a Blue Strain antigen, however, by this time, the much deadlier Red Strain had been spotted.