The Virus X story ran from Action #362-366 in early 1968. It starts when Ventor, a stage ventriloquist, tries to get revenge for his brother Bruno, who died in prison after being captured by Superman. Ventor kidnaps Clark Kent and hypnotizes him into hating Superman and being determined to kill him.
In the second story Luthor gives Ventor a test tube of Virus X, which is so powerful that it kills earth creatures almost instantly. It contains just enough Kryptonite to make it effective against Superman. Clark Kent starts to pour the virus on the bed where he knows Superman sleeps, and accidentally gets some on his own hands. As it works out, this restores his memory. Unfortunately it's too late as his hands have turned into green claws, showing he has been infected.
In the third instalment, the worlds best doctors and scientists are unable to cure Superman. Luthor claims to have an antidote, but he turns out to be lying. In the end, Superman places himself into a clear coffin and takes off for the heart of the hottest sun in the universe.
The fourth story is something of a retrospective. As Superman flies through space, the story of the entire Superman legend is retold in his memory. We learn of the death of Krypton and baby Kal-El's raising by the Kents, and his romances with Lana Lang, Lori Lemaris and Lois Lane. Even the Bizarros get into the act, strewing White Kryptonite in the path of his casket.
In the final story, it is revealed that the White K cured Superman by killing Virus X, which was a form of plant life. But when Superman returns to Earth, he discovers that somebody has taken his place. So the mystery becomes who has done it. Unfortunately DC gives it away in the splash page, which features the members of the Justice League of America. In the end, Superman is able to resume his role as Earth's mightiest hero. This leaves the loose end of Ventor, who has never been punished for his effort to kill Superman, as a letter writer points out in Action #369.