Royal Raymon Rife (May 16, 1888 – August 5, 1971) claimed to have used a special optical microscope to observe very small viruses. Bacillus X and Bacillus Y (at some point renamed BX and BY viruses) were experimentally isolated, and Rife causally linked them to two forms of malignancy (cancer): carcinoma, and sarcoma. This renaming of these bacillus forms by Rife to that of viruses is now known to have been in error. The limitations of optical microscopes, and the size of viruses is such that most viruses cannot be seen under an optical microscope. Furthermore, the scientific understanding is that the estimated 15% of human cancers that are caused by viruses are caused by a number of different types. Rife's virus claims must therefore be seen as mistaken.