The M-Virus is the more common virus, which turns humans into The Infected, turning them into a new, different species. It can be transferred from the Infected to humans by bites, scratches or any instance where the blood of an Infected enters a human's system. It is this virus that is plaguing the majority of the world due to its extremely fast conversion rate. After being bitten, symptoms appear in a few seconds, and never later than twenty-four hours.
According to Dr. Yukko Sakagami's husband, the M in the name stands for Mother.