Johne's (pronounced "Yo-nees") disease is a contagious bacterial disease of the intestinal tract. A German veterinarian first described the disease in a dairy cow in 1895; his name is used as the common name for the disease. The disease is also called paratuberculosis.  Johne's disease occurs in a wide variety of animals, but most often in ruminants. The bacterium that causes Johne's disease is named Mycobacterium paratuberculosis, often the name is abbreviated M. paratuberculosis.

Primarily, there are only two signs of M. paratuberculosis infection: diarrhea and rapid weight loss. In goats, diarrhea is less common. In general, animals with Johne's disease "waste away" despite their continuing to eat well. Infected animals maintain a normal temperature but may appear unthrifty and can become weak in later stages of the infection. Because of the slowly progressive nature of the infection, signs of Johne's disease are usually not seen until animals are adults. Since the signs of Johne's disease can be confused with the signs of several other diseases, a diagnosis can be confirmed only by use of laboratory tests.

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. When using laboratory tests for pre-purchase screening of animals, it is important to understand that tests done on individual animals are not 100% sensitive, meaning they can't detect 100% of all infected animals. A way to get around this problem is to rely on tests done on the source herd of animals from which you want to buy. If a whole herd test is 100% negative, then the probability the herd is free of M. paratuberculosis infection is very high. Johne's disease test-negative herds are the best sources of animals for purchase.

Methods for Johne's disease control depend on the type of animal and the patterns of husbandry. In principle, two strategies must be employed at the same time:

• Newborn animals must be protected from infection by being born and raised in a clean environment and fed milk free of M. paratuberculosis.
• Adult animals carrying the M. paratuberculosis infection must be identified by laboratory tests and removed from the herd, flock or enclosure.