Originally a menial laborer, Rudy Jones was transformed into the Parasite while working as a janitor at the Pittsburgh branch of S.T.A.R. Labs. Unknown to anyone at the scene, Darkseid, the lord of Apokolips, remembered the Parsaite who had existed prior to the Crisis on Infinite Earths (Maxwell Jensen) and manipulated events in order to turn Jones into a new version. Under Darkseid's influence, Jones opened a waste container believing it contained something of value he could steal, and was exposed to a strange form of radiation which changed him into the bald, green skinned Parasite. Jones now had the ability to absorb the life energy of others, leaving behind a smoldering skeleton. This power was necessary for his survival as his own body is in a constant state of hunger for energy that it can't sustain on its own.
More on this character soon, for the meantime read his bio here: