This isn't a virus or a weakness issue. I thought it was too, when I went through this last year. More then likely its your feed. Take them off of it and feed them nothing but alfalfa hay for a couple of weeks. Get them some nice stemmy stuff. If they improve and you don't see any more deaths during that time, then its your pellets causing it. If the pellets are corn based, get them off of them and get them on something else that isn't. When we had issues last year there were corn toxins found in some of the feeds. It was causing e coli and all kinds of nasty stuff. Also get these rabbits on some acid pak 4 way pro bios. Some of the rabbits that were lost last year dehydrated themselves to death, while others suffered from bloat. All the problems went away when I switched to a non corn based feed. It was purina rabbit chowe at the time. Don't feed that one though, because they changed the formula and its corn based now. Go with the show or fiber 3. other brands we had issues with were manna pro, kent, and pen pals. they claim they are not corn based. they really are. Its in the grain by products they use.