It is believed that the vast majority of M. paratuberculosis infections in cervids occur while they are calves. As they mature, it is believed that they become less susceptible to infection. Complete resistance is unlikely however as adult deer/elk can acquire the infection if given a sufficiently large dose of M. paratuberculosis at a period of immune insufficiency. Infected adults serve as the source of infection for calves as they shed the organism in manure, milk and colostrum. The contaminated manure may then become concentrated in standing water and wallows.		This contagious mycobacterial disease spreads from infected adults to the most susceptible age group (calves) by a number of routes. Although less susceptible than young animals, adult deer/elk can also acquire the infection. The organism can be shed by an infected animal into milk and manure; the manure then contaminates water, wallows and solid feed. Cervids can acquire this infection from cattle and likely other infected species, such as bison, sheep and goats.

Most calves acquire the organism by suckling from manure-contaminated teats, by licking contaminated flooring/fencing/feed bunks and waterers, by drinking from a contaminated wallow or by eating off of ground contaminated by contaminated manure. Since M. paratuberculosis is thought to be excreted in the milk of infected lactating deer/elk, as has been shown to be the case in cattle, calves can become infected by nursing from or being bottle-fed milk from an infected dam. The risk that the organism is shed into the milk is believed to be higher in animals that have moved into later stages of the infection. Thus clinically affected deer/elk (as shown by weight loss and perhaps diarrhea) are more likely to infect their offspring than animals still in good condition.

Another, but less common, route of exposure occurs in utero. Again during the later stages of infection, the organism can disseminate beyond the initial location of the infection (i.e. the ileum, a portion of gastrointestinal tract). At that time, if the animal is pregnant, the fetus can also become infected. This infected and infectious calf appears completely healthy at birth, although spontaneous abortion of fetuses with this infection has been reported in cattle.

There are different strains of M. paratuberculosis, some of which apparently infect certain animal species preferentially ("sheep" vs. "cattle" strains). It is likely however that all M. paratuberculosis strains are capable of infecting deer/elk and any strain found in deer/elk can be transmitted to any other hoofstock species. Thus animal managers should consider the Johne's disease status of any hoofstock in contact with cervid herds.