Giant viruses aren't the result of nineteen-fifties Atomic Mutation (now playing in our air-conditioned theater!) but a real infectious agent so unexpected they were only discovered recently. A giant virus was like a tiny elephant - something someone would not only not expect, but actively miss every time they looked because their initial assumptions would screen them out. Now another massive virus has been discovered.
The Marseillevirus (go on, guess where the discovering institute is located) is the fifth largest virus ever found and does things to genetics that make The Fly look like a faithful photocopy. It evolved inside amoebae, which - as well as oozing around the place doing their unicellular thing - act as hosts for an entire ecosystem of infectious agents. Viruses evolve particularly rapidly, stealing genes from whatever host (or even other viruses) they can get their proteins on, experimenting with millions of variations to find which ones work.
Some viruses become so large that other viruses can infect them, tiny super-specialized parasites piggy-backing their genetic code on their huge cousins. Others simply fail, their patchwork genomes not up to the challenges of the environment. The Marseillevirus has stolen ten percent of its chromosomes from bacteria it once infected, and anther five percent from the amoebic host of the party. It's even infiltrated other giant viruses, taking parts of the mimivirus for its own purposes.
Don't worry about any mass-media scare stories: this thing is no threat to us and there isn't a shred of evidence to suggest otherwise. Not that the latter necessarily affects the former.