Ambrosia

Ambrosia is a vaccine for the Gray Death. It does not cure the plague directly, only neutralizes it. One dose (20cc) works for 48 hours. Afterwards, the patient is still in danger and effectively dependent on the supplier.

Ambrosia is described as "non-organic compound containing eutactic components". It can only be manufactured nanotechnologically, with Universal Constructor. For that reason, it's available in so small quantities, that only the richest can afford it. In Greek mythology, Ambrosia was the food of the gods, which gave the immortality. Analogy is clear.