Virus X

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 viruses—if, indeed, any survived the destruction of Superman's native planet—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 have been 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 discovered that he could destroy the virus with "element 202." Because element 202 is fatal to human beings, however, Tharb-El was unsuccessful in his efforts to produce a viable cure (S No. 156, Oct 1962: "The Last Days of Superman!" pts. I-III—"Superman's Death Sentence!"; "The Super-Comrades of All Time!"; "Superman's Last Day of Life!").