Periodic fever is an uncommon condition defined by recurring bouts of fever in people who seem healthy except during their fever attacks. The condition is hard to diagnose because there are several types of periodic fever as well as multiple causes of recurring fevers. In 1984, researchers identified a type of periodic fever that they called the hyper-IgD and periodic fever syndrome (HIDS). People with HIDS have persistently high levels of an antibody known as immunoglobulin D (IgD). They have recurring bouts of high fever with swollen lymph nodes, abdominal distress (vomiting, diarrhea, and pain), and joint pain. Symptoms last for several days and recur every few weeks. Researchers think that the underlying cause of HIDS is a mutation of a specific gene (the mevalonate kinase gene). This mutation can be identified with special molecular genetic tests. Whether the tests can be used to diagnose HIDS is unclear, however.