The Dutch Flu (In Game Virus) is a disease/virus that started the zombie infection in earth. It was first Identified as a weird tulip eating cult, however, Netherlands exported the virus not knowing that they have been exporting the zombies. Later, the virus was found contagious but was not contained and it further infected living humans. It wasn't able to infect animals even though there were no animals seen throughout the game except for Dogs and Cats.
The Dutch Flu is described as a Weird Tulip Eating Cult as said by the protagonist, only in Rebuild 1. It could've been contained when it was already known, but it has been brought to different countries, sending the virus all over the world, it infected the whole planet leaving nothing behind but small groups of survivors. As said by the main character, it came from the Netherlands.