Johne's disease typically enters a herd of animals when an infected, but healthy-looking, animal is bought. The infection then spreads to other animals without the owner's knowledge. Eventually, often after several years, the owner recognizes signs of the disease in a number of animals. Individual animals get infected by close contact with other infected animals. Most often, the infection is acquired by eating material contaminated with M. paratuberculosis when animals are very young.
Young animals are far more susceptible to infection than are adults.
Ingestion of the bacterium occurs when the newborn's environment is contaminated with manure from an infected animal, or by drinking milk from an infected animal. The bacterium passes out of infected animals primarily in the feces, but in the later stages of the infection it is also found in the milk of dairy cattle and presumably the milk of other animals too. After infection, many months or years go by until the infected animals shows signs of Johne's disease. The best way to avoid this chronic infectious disease is to be as certain as possible that animals brought into the herd are not infected with M. paratuberculosis . This is not always easy.