A highly contagious and painful disease capable of killing millions, whose outbreak on Styris IV sends the Enterprise-D to Ligon II for the only effective vaccine.
Anchilles fever is the emergency behind the Next Generation episode Code of Honor. The disease spreads fast and brings a slow and agonising death, and the Federation world Styris IV faces casualties in the millions.
The vaccine cannot be replicated; a sample analysed by Beverly Crusher proves unstable, so the real thing must be obtained from Ligon II. This gives the Ligonian leader Lutan a strong bargaining position when he abducts Tasha Yar.
Picard has to respect Ligonian custom to the letter, including a ritual fight to the death between Yar and Lutan's wife Yareena, in order to secure the shipment.