In the late-19th century, scientists showed that certain infectious agents, such as those causing tobacco mosaic virus and yellow fever, were distinct from other microbes because they were so small. Still, it was presumed that they were living organisms until 1935 when tobacco mosaic virus was crystallized. The discovery of its acellular structure made viruses "seem more like nonliving chemical entities of disease," a view still held by many, writes Luis Villar-real of the University of California, Irvine, in an E-mail. Most definitions of life include the processes of metabolism, growth, and reproduction, which viruses cannot do on their own. But viruses can employ the machinery of the host cell to perform these functions. Further, viruses evolve over time, satisfying the long-term adaptation requirement of living things.