10 Ways To Beat Superman

Virus X first appeared in the 1962 story The Last Days of Superman. Superman found a satellite from Krypton, only to discover that it contained a sample of a deadly virus fatal to all Kryptonians. Feeling weak and fearing that he may be dying, Superman quarantined himself and drew up a last will and testament.

Then it turned out he was actually just feeling ill because the entire satellite was made of krytonite. There's a reason super-intelligence isn't usually listed as one of Superman's abilities.

A few years later, a completely different version of Virus X appeared. This time it was a kryptonite based virus made by Lex Luthor (who wanted to prove that humans can be just as uncreative at naming things as aliens).

In the kind of plot that you only get in SIlver Age comics, Clark Kent was hypnotised by an evil ventriloquist into trying to kill his best friend... Superman. While hypnotised, Clark ended up infecting himself with the virus and was kicked out of Metropolis once he got all green and lumpy.

Tthe effects of Virus X were eventually undone by white kryptonite because coming up with story ideas is hard and writers need their naps.