Calves born to Johne's-infected cows, are 10 times more likely to be infected than are calves born to non-Johne's cows in the same herd.
Studies done back in the '80s, following high fecal-shedding cows to slaughter, showed that M. avium subsp. paratuberculosis, the bacterium that causes the infection, could be isolated from fetal tissues in at least 25% of the animals.
So...potentially at least 25% of calves out of Johne's cows are BORN infected. The organism is shed in feces, so any fecal contamination/ingestion is a risk - and we also know that Johne's cows also shed the organism in colostrum and milk, so even if calves 'escape' in utero infection, they are at risk from ingesting the organism in colostrum/milk, or from feces on the dam's udder - and throughout the environment.
Studies have also shown that the organism can survive for extended periods - over a year - in the environment, especially in moist, damp areas around waterers, feedbunks, etc. Soil pH also seems to have some effect on environmental persistence - acid soil conditions favor survival of the organism.