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, nether lands exported not knowing that they have been exporting the zombies. Later in days and weeks, the virus was found contagious but was not contained and it further infected living humans. But it wasn't able to infect animals but there were no animal 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 in Rebuild I . However it could've been contained when it was already known, but it was exported 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 country, Nether Lands.