Viruses are infectious agents that have no organelles or reproductive machinery of their own. Viruses cannot duplicate their DNA or RNA, nor can they translate their genetic information into protein. Essentially, they are small bags of genes that typically encode a comparatively small number of proteins. For example, the human immunodeficiency virus (HIV) is composed of only nine genes, yet with these simple nine bits of protein it can wreak havoc on the human immune system. Others, such as herpes simplex or adenovirus, can have large genomes with dozens of genes. Simple or complex, though, all viruses have the same function. As they cannot make protein or reproduce on their own, viruses must force bacteria or animal cells to do their work for them. A virus is, simply put, a genetic parasite.

As it does not have to sustain other energetically expensive cellular processes, a virus has a...