Raymond Maxwell Jensen was a janitor with greedy ambitions. When he learned that the laboratory he was working in had hazardous containers stored within, he suspected they were secretly used to store funds. However, the container was realy storing radioactive materials from outer space which Superman was experimenting on. By opening this hazardous container, Jensen's flesh was seared and he was transformed into an atomic furnace. Capable of absorbing and burning through energy at rapid rate and in massive quantity, Jensen also consumed the intelligence of the resident scientists, and learned that as the Parasite he would need to leech a power source greater than mere humans to survive. His target was Earth-One's Superman, and upon discovering his alter ego, Jensen adopted the alias of reporter Larcon Leech. Gaining employment at the Daily Planet, Leech was able to leech powers off Superman gradually. Eventually, his scheme was revealed leading to his imprisonment.
Heinrich Melch was a solider in Germany during World War 2 with revenge in his heart. When his father and another scientist discovered how to liquefy radioactive Kryptonite meteorites, they injected it into Melch's blood stream making him an atomic furnace. This provided him superhuman abilities harnessed from the energy, while gaining the ability to drain the life from his foe simultaneously Travelling to America at the conclusion of the war, Melch adopted the alias of reporter Henry Miller, obtaining employment at the Daily Star in order to close in on his prey Superman who had an association with that newspaper. After a series of confrontations versus his heroic opponent, Melch as Atoman was finally defeated despite having severely weakened Superman. Thinking Atoman was dead following this final battle, Superman buried Melch in a lead lined coffin while cladding the villain in his hooded costumed. However, Atoman survived and struggled to revive himself for decades to come.
Parasite would appear to have perished more than once although he gained the ability to rprotect assemble his atomic mass. On one occasion, he came into conflict with fellow felon Lex Luthor, as both were in a competition to defeat their common foe first. Later, Jensen adopted the identity of Superman's long lost brother, using power that he stole from his enemy while attempting to frame him in the court of public opinion. With Batman's assistance, Superman was able to expose this sinister scheme. Some of his other malevolent machinations included spawning a clone of the Solomon Grundy from Earth-Two to use against the Man of Steel and other subterfuge. Concurrent to these events, Parasite developed a Power Prism which like his own inherent abilities was capable of drawing energy from individuals and transferring them into others, such as himself.
A similar object known as the Power Stone, originating from outer space, likewise could pull energy from some and imbuing it upon others. This allowed first Alexei Luthor of Earth-Two, and shortly thereafter the Ultra-Humanite, to gain superhuman abilities matching their mutual foe Superman. As to Atoman, he finally revived himself decades after he seemingly perished, still feeding upon the radiation coursing through his veins. Now on Earth-One, Atoman sought revenge against the man whom he thought vanquished him and his people long ago. However, that world's Superman alongside Batman eventually figured out where Atoman was truly from, and brought Atoman back to Earth-Two where the pair as well as the original Superman and Robin defeated him for good.