Of the hepatitis alphabet, the C variant may be the nastiest. In 1990, researchers observed that most patients with hepatitis C also develop a rare autoimmune disease called mixed cryoglobulinemia, a condition that frequently leads to cancer, arthritis or both. Now, researchers say that a decade-old explanation of how one disease causes the other is likely wrong, and instead offer a new — albeit controversial — theory of their own: that the pathogen causing the disease zeros in on a specific cellular target that has yet to be identified.