Animals are the most important sources of bacterial infection for humans. Whether from pets or undercooked meat (especially pork), these bacteria almost always enter the human body through the mouth (oral transmission). An incubation period of one to eleven days passes before signs of disease develop. Rare cases have been transmitted by way of contaminated blood transfusions.

Yersinia produces several different types of disease. The most common form is a short-lived inflammation of the intestine known as enterocolitis. Most often the very end of the small intestine is involved, an area known as the terminal ileum. The result is gastroenteritis, with cramping abdominal pain, fever, and diarrhea. Diarrhea generally continues for two weeks or so, but can go on for many months. Up to 40% of patients also experience nausea and vomiting; and in one-third, inflammation of the intestine leads to bleeding.

In other patients, the same area of the intestine is involved, but instead of causing diarrhea, a syndrome resembling appendicitis occurs. In this syndrome, the lymph nodes surrounding the intestine are especially involved; this has lead to the term mesenteric adenitis. Although this syndrome resolves without serious consequences, it is often difficult to differentiate from appendicitis, and leads to surgery in some instances. Ultrasound exam may be able to demonstrate a normal appendix and avoid surgery. Why some patients develop symptoms of gastroenteritis, and others only inflammation, pain, and fever, is unknown.

In some patients, Yersinia produces infection of areas other than the intestinal tract. These include:

• Inflammation of the throat (pharyngitis) and tonsillitis; this can be quite severe and even lead to death, particularly in adults.
• Septicemia, or infection of the blood stream, with spreading of infection to other organs such as bone, meninges, kidneys, and others. Individuals with decreased immunity due to liver disease, diabetes, cancer, and other diseases are at increased risk for this complication.

Different parts of the body may be affected (such as joints, eyes, and urinary system) by changes in the immune system caused by Yersinia infection. Arthritis, which is especially frequent in Scandinavia, occurs in up to 10% of Yersinia infections. About one week after typical intestinal symptoms, swelling and pain in multiple joints occurs. The knees and ankles are most often involved, and become inflamed over a period of two weeks. In two-thirds of those affected, symptoms gradually resolve over one to three months without need for treatment. Rarely does chronic joint disease develop.

Inflammation of the heart muscle, called myocarditis, sometimes occurs together with the arthritis. In about 15-20% of patients, the skin develops a red, raised area, usually located on the shins, called erythema nodosum. This appears within a few weeks of the intestinal symptoms and disappears over a month or so.