And if viruses are chemicals, what is the nature of parasitic chemicals that replicate in a living cell? Viruses infect all kindoms* of life, and some ‘satellite’ viruses even require other viruses for replication[1]. Generally, plant viruses, termed viroids, are just naked loops of RNA, bacterial viruses have a protein outer shell, and animal viruses wear a coat made of cell membrane from the last cell they infected. The second hypothesis is the Progressive Hypothesis, which posits that viruses evolved from mobile genetic elements in the genome. In nature there are giant viruses that are so enormous in size that they are sometimes termed girus (from giant virus).