What is the cause of left 4 dead?

Most humans who came into contact with the <b>Green Flu</b> were converted into zombie-like beings known as the Infected.

Question

Most humans who came into contact with the Green Flu were converted into zombie-like beings known as the Infected.

Hagrid P Hazard · Accepted Answer

Most humans who came into contact with the Green Flu were converted into zombie-like beings known as the Infected.