A silicon-based viral illness that could have been cured using a culture grown in an active positronic matrix, unavailable after the Federation's ban on synthetic life, and which killed Thaddeus Troi-Riker.
Mandaxic neurosclerosis is a rare disease caused by a silicon-based virus. In principle it is treatable: the cure requires a culture grown in a functioning positronic matrix of the kind found in a Soong-type android.
The illness is described in the Picard episode Nepenthe. Thaddeus, the son of William Riker and Deanna Troi, fell ill with it in the years after the synthetic attack on Mars, when the Federation had outlawed synthetic life and all research into it. No active positronic matrix could be had, so the one treatment that would have saved him was out of reach.
Riker and Troi left Starfleet service and settled on the planet Nepenthe, whose soil was reputed to have regenerative properties, hoping it would help. Thad died there, leaving his parents and his younger sister Kestra, who preserves the languages and the imaginary homeland he invented.