I don't really know the disease name in English, but it is a kind of virus that will cause high percentage of dead cases especially in young rabbits.
The virus will spread using Lytic Cycle method. Those who study science should understand what I mean. Those viruses will breed in rabbit's intestines and increase its population. When it's time to spread around, they will cause intestine to burst. Poor rabbit will die without any symptoms. As we all know, viruses that breed by Lytic Cycle will kill the host as well. In this case, the host in the rabbit.
Poor things :(