Except under unusual circumstances infection begins in the first few months of life. Animals become more resistant with age, though resistance is never complete. If given a sufficiently large dose of the bacteria adults can become infected. Nevertheless, under normal animal husbandry conditions, exposure to such large doses probably rarely occurs. This means, for example, that an infected adult dairy cow in a stanchion or tie-stall barn is not going to infect the cow standing next to her.
Infected cows are primarily the main source of infection for calves.
In a beef herd, large doses of pathogens may be deposited on ground-fed forages thus increasing the risk for other animals and especially youngstock. The vast majority of infections in young animals are acquired by ingestion of the bacteria. This happens accidentally when they consume manure containing the bacteria. Suckling manure contaminated teats, licking contaminated bars in the stall where they are born, or being housed in a location where they have access to manure from the adult herd are ways young animals have opportunities to ingest this microbe. Johne s disease typically enters a herd when infected, but healthy-looking, animals come in.