Amazingly, it could be a miniscule microbe that could finally spell doom for the Man of Steel, leading into...."The Last Days Of Superman", the comic being covered today.
Superman #156 (October, 1962) was by Edmond Hamilton, Curt Swan and George Klein, with Swan/Klein giving us the cover.
Superman's Death Sentence
Going to an Earth doctor, the doctor says he has no way to check Superman in any meaningful manner, but that his increased heart rate and sudden weakness lead him to believe Superman has less than 30 days of life left. Superman leaves the hospital and his Daily Planet friends, and contemplates how to better the world, enlisting Supergirl's help to gather others, as Superman has his robots construct an isolation chamber around him (as Superman feels weak when talking to Jimmy Olsen).
The Super-Comrades Of All TimeLegion of Super-Heroes (except for Brainiac 5, who mysteriously won't join in) and the Kandorian Superman Emergency Squad to perform many duties to make the Earth better and to keep the planet safe from threats that will menace the planet after Superman is gone, Superman gets another bout of weakness as Jimmy Olsen and Lois Lane visit Superman in his self-imposed exile in the desert. The heroes find ways to stop a rouge planet from eventually crashing into Earth as well as stopping a cloud of space fungus that would kill all plant life on Earth should it have reached our planetary orbit. All this activity has stressed out poor Lois Lane, who faints from the pressure, with Jimmy Olsen taking her back to Metropolis in the Flying Newsroom, only to come back to find Superman has left his cell.
Annoyed by the lack of participation by Brainiac 5, Supergirl heads back to the future to confront the intelligent Legionnaire, finding he had not turned to evil like his ancestor (and Superman foe, Brainiac), but instead was working in the lab to find a cure...but was unable to find one on Earth.
Superman's Last Day Of Life
Wait....Tharb-El killed the sample before it reached Earth? Then, why is Superman dying? Mon-El (at the time, a silent resident of the Phantom Zone) provides the answer....a small piece of Kryptonite had logged in Jimmy Olsen's camera, and made Superman sick every time Jimmy came near (the virus was never active or infectious). Finding the Kryptonite, Superman recovered (and Supergirl and Krypto erased Superman's alter ego message on the moon).