curlygirl. All cattle carry good bugs & bad bugs. They become "immune" to the bad bugs "within a herd" through the colostrum from their dam. But every herd has differend "bugs". So you're new born baby had all the immunities it needed to avoid getting sick from bugs her dam had. But, you purchased "healthy" calves that had their own set of bugs that they were immune to. But, when you mix HERDS' bugs, they don't have the immunities built up to combat the bugs from the unknown herd.
Not sure if that made sense.
Also, within a herd, if a calf is super stressed or super saturated with bad bugs, they don't always have ENOUGH immunity built up to combat them. Like - born in dirty calving lot, or severe weather change (cold wet) or poor nutrition. Then they also will get sick - which is usually diarhear or respiratory.