Scientists have argued for hundreds of years over how to classify viruses, says Luis Villarreal, professor emeritus at the University of California, Irvine, where he founded the Center for Virus Research. In the 1700s, viruses were believed to be poisons. In the 1800s, they were called biological particles. By the early 1900s, they’d been demoted to inert chemicals.
Throughout, viruses have rarely been considered alive.