The decoding of the human genome brought one huge surprise that remains unknown to most people, outside a small field of geneticists. That about 8% of our DNA does not originate from human ancestors, but from viruses. These viruses have accompanied living creatures since the beginning of evolution, and developed a mechanism to inject their genetic material into that of their hosts. Once injected into mammalian sperm and egg cells, these retroviral insertions travel with their hosts through millions of years. Biologists knew about this process for a long time, but dismissed its significance because over the course of eons, the integrity of these “endogenized retroviruses” disintegrated. But recent work now shows that even fragments of these “human endogenized retroviruses” – HERVs – can affect the function of human genes. Even more astonishing, some of these HERVs are required for life as we know it. In fact, humanity could not exist without them.

In my talk, I will tell you the fascinating story of these HERVs. I will tell you about tantalizing evidence that comes from modern cancer drugs, schizophrenic patients, and patients with multiple sclerosis. I will tell you how the virus “within” may protect us against the virus “out there”, and how HERVs may solve the mystery of “missing heritability” in mental illness. I will tell you how stress activates molecules that can either protect us against pathogenic HERVs or make things worse. At the end of our evening, you may walk away from this talk with a new sense of what it means to be human in a viral world.