In left 4 dead how did the infection start

The zombies you see in Left 4 Dead 1-2 are infected with a rabes-like disease that makes them very destructive and have a desire to infect everything that is healthy. The zombies have already found out that you (The Survivors) have an immunity to the disease that they carry, so they decide to kill you instead.

This disease apparently randomly came into action on the Earth and since then has infected over half of the world's population.