This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
Mendaxic neurosclerosis (MN) was a very rare disease caused by a silicon-based virus. It was fatal if not treated, which could be accomplished by culturing infected cells in an active positronic matrix.
In the late 24th century, Thaddeus Troi-Riker contracted mendaxic neurosclerosis, which prompted his parents William Riker and Deanna Troi to settle on Nepenthe. Thaddeus eventually died of the disease, as there were no positronic matrices available for his treatment due to the Federation ban on synthetic lifeforms. (PIC: "Nepenthe")