This deadly Kryptonian virus, for which no cure has ever been discovered, is described in Superman No. 156 as "a contagion fatal in 30 days to any native of Krypton...." Because living X virusesâ€”if, indeed, any survived the destruction of Superman's native planetâ€”would acquire super-virulence in the alien environment of Earth in the same manner whereby Superman acquired his super-powers, Superman and all other surviving natives of Krypton are vulnerable to this killer virus just as they would be had Krypton never exploded and they, and the virus, remained on Krypton.
In his experiments with Virus X prior to the death of Krypton, the Kryptonian scientist Tharb-El discovers that he can destroy the virus with "element 202." Because element 202 is fatal to human beings, however, Tharb-El is unsuccessful in his efforts to produce a viable cure (S No. 156, Oct 1962: "The Last Days of Superman!" pts. I-IIIâ€”"Superman's Death Sentence!"; "The Super-Comrades of All Time!"; "Superman's Last Day of Life!").
Another Virus X story is chronicled in Act No.s 362-366 , Apr-Aug 1968: "The Head of Hate!"; "The Leper from Krypton!"; "The Untouchable of Metropolis!"; "Superman's Funeral!"; "Substitute Superman!". In this adventure, the effects of the virus are depicted as similar to leprosy on Earth: the skin of the victim becomes bloated, distorted, and green-colored. Unlike the 1962 account, Virus X is not only fatal to Kryptonians but highly contagious and fatal to Earth creatures as well. Superman is eventually cured by accidental exposure to White Kryptonite (thrown by Bizarro Supermen at his funeral spaceship) which, being lethal to all plant life, destroys the virus within Superman's body.
(see also Flammbron)