Time for the whole outbreak story:
Scorpion 7 was researching a new virus as a biological weapon for the U.S. government. Somehow, the Scorp- facility on Washington got a leak and infected all people there. The US Army reacted fast and send a group of special forces to the facility. The operation went wrong and caused the outbreak on the Washington map.
Later, an infected person got from an unknown airport on a plane with the destination Russia. During the flight the person infected all passengers and the crew. This means that the plane was landed by autopilot. After the landing, all persons in the airport were evacuated to Moscow, spreading the virus there. The Russian government gave the order to nuke Moscow, killing everyone in the city. The responsible general manipulated the rocket because he “doesn’t want to kill thousands of innocent civilians” as he wrote in a note. The radiatoin of the nuke caused the deathzone in the north of Russia. Instead of this, the subway tunnels were destroyed, in the hope of getting the infection trapped in Moscow. However, a train managed to get out with infected on board, spreading the infection to St. Petersbourg. From threre, the infection rolled over the country. All the Russians could do, was to close or destroy all tunnels to other countries and build a savezone on an oilrig which got radiated and infected later. Nowdays we have the liberator savezone. Between the infection of Russia and Germany was quite a bit of time since Germany had the time to build walls and founding a space program which seemed to be used for evacuating people into space.But the barriers where poorly and bad guarded like one of the guards wrote in a note: “if something wants to get over this walls, it will”. After the outbreak in Germany, all efforts of the German Armed Forces and the Coalition to fight the outbreak failed or got sabotaged by the “Syndicat”.
How the virus was spread to the other maps is unknown but I think that it was bought there by refugees from the infected countries. But this is just speculation, for more we would need more information about the virus: is it contained by animals or water? Do you get it by direct body contact with infected?