This is going to sound a little strange, but this last calving season we ran into a couple or so calves that got pneumonia type symptoms and died before they were maybe 4 days old. Tissue samples showed a type of bacteria (I think that's correct) that the cow carried and the calf actually had the beginnings of a deadly pneumonia before it was ever born. The cure was to cull the cow. If nothing else, remember this and see what goes on with this cows next calf if you keep her.