Just how small can parasites get? The final layer of these systems might be the transposon, which is a roving bit of nucleic acid—a single, parasitic gene. Transposons have been discovered inside viruses that infect other viruses, which in turn infect amoebas that infect human beings. "I think it's difficult to see where one organism begins and another one ends," Bordenstein says. "We are only beginning to appreciate how intertwined these layers of organisms are in large flora and fauna."