I have an operation of 45 breed-able cows. Twice this season a calf is born and looks healthy. 3-4 days later I find it dead in a field. No marks, looks hydrated, everything is fine but it's dead.
Next time you have a calf die out of the blue. I would consider you call you vet and get a post op done, the vet could take tissue samples from the dead calf and be able to help determine the cause of death to help in the future, when choosing a vaccine for your herd.