Campylobacter jejuni: A bacterium that typically infects the bowels. Now a leading cause of bacterial food poisoning, Campylobacter jejuni is most often spread by contact with raw or undercooked poultry. A single drop of juice from a contaminated chicken is enough to make someone sick. Symptoms tend to start 2 to 5 days after exposure and typically last a week. They resemble the symptoms of viral gastroenteritis'diarrhea, fever, abdominal pain, cramping, nausea, and vomiting'but with campylobacter, fever is typical and the diarrhea is often bloody. Antibiotics can be helpful treatment. Most people recover completely. However, some suffer long-term consequences, such as arthritis or Guillain-Barr' syndrome. Both are thought to occur when a person's immune system is activated by the Campylobacter jejuni and misdirected to attack the person's own body.