AT: "mu" Ulcartic virus was a very deadly and very painful disease, sometimes used as a brutal method of execution by the Klingon-Cardassian Alliance in the mirror universe. The virus caused the subject's internal organs to heat up, to the point where the victim's body literally boiled to death.
Elim Garak planned to execute Intendant Kira Nerys with a concentrated dose of ulcartic virus in 2375, but when Regent Worf spared her life, Garak planned to use it on Zek, Quark, and Rom instead. However, Ezri got the drop on Garak and injected him with his own hypospray, killing him. (DS9: "The Emperor's New Cloak")