I know of a couple of bunnies that were bitten by rats and got ill from it, one had to be put down the other was fine but just seemed 'odd' apparently.
However, when I was a kid we kept the rabbits in the stables, and then the stables got infested with rats when they dug up the sewers accross the road. It was horrible and we must have had loads, if you went into the stable in the evening you saw/heard them shoot up the walls as they heard you coming.... urgh. But none of the 20 or so buns were hurt, although we did find that one mummy rabbit had killed one once, we found it dead near her nest, we're not sure if it attacked her litter (she rejected the litter, which unusually was just one baby). But I dont think they generally actively attack rabbits, or we would have had lots more problems...
edit: do you have a dog? We never had rats in the house because they could smell the dogs, even though the whole street got them in their houses, so it may be an idea to walk a dog round the shed to try and deter it.