This study will try to identify new or known viruses responsible for diseases or conditions of unknown cause. Viruses are an important cause of illness. They have been identified as causes of several cancers, including certain liver cancers and cervical cancer. Several diseases, such as Kawasaki's disease, aplastic anemia, aphthous ulcers, and certain fevers of unknown origin in immune suppressed patients are thought possibly to be due to viruses, but their cause has not yet been determined. This study will use very sensitive molecular techniques to try to identify new or known viruses in patients with conditions of unknown causes. This study does not involve any treatment.

Any patient with a condition or disease whose cause is unknown may be eligible for this study. A referral from a local physician is required for patients who wish to enroll in this study.

Participants will have blood samples drawn up to four times during the study. The total amount of blood drawn in any 6-week period will not exceed 50 milliliters (about 3-1/2 tablespoons) for adults and 5 ml (about 1/2 tablespoon) per kilogram (2.2 pounds) for children. In addition, body fluids, such as joint or spinal fluid, or tissue samples, such as biopsy specimens, obtained previously for medical purposes, may be obtained for testing.

The patient's doctor will be informed of any positive findings from the samples. Identification of a virus does not necessarily mean that the virus is causing the patient's condition or disease. Additional tests from many patients with similar diseases or conditions are required before a virus can be definitively associated with a disease. Similarly, failure to find a virus does not necessarily mean that the patient is not infected with a virus that is causing the condition, as the tests used in this study are imperfect and will not detect certain forms of viruses that can cause disease.