You are at least 8% virus! The human genome contains roughly 20,000 protein-coding genes whose coding sequences comprise only about 1.5% of your 3.2 billion base-pair genome. It turns out that over the course of evolution, the lineage leading to humans experienced numerous retroviral insertions into germ line chromosomes. Most of these remnant viral sequences are non-functional today, but we carry them as molecular fossils. However, some genes introduced into our genome by viruses were co-opted for new functions, including a gene that makes the protein syncytin, an essential player in placental development. Did viruses make mammalian evolution possible? It looks like it. In my Viral Genetics class this semester, we will be investigating the diverse ways in which viruses manipulate the genetic processes of their host cell, the role that viruses play in disease and the ways in which viruses have shaped the biology of other organisms over evolutionary time. We generally think of viruses as pests and parasites, but would we exist without them?