October 27th, 2012, 03:34 PM
Viruses are not considered to be alive because they are unable to independently replicate. It seems most likely that viruses are bits of nucleic acid that became detached from a cell and became able to enter and replicate in other cells.
Bacteria routinely swap bits of DNA called "plasmids." Probably the earliest form of sex. And from that, it's not hard to see how a plasmid could become parasitic on cells.
There are even simpler things called "prions" that are merely proteins, but they can replicate themselves in cells. They are infectious, and cause a number of diseases like mad cow disease. They aren't considered alive, either.
There are a number of man-made replicating chemical systems that are more "alive" than either viruses or prions.