Understanding viruses: Philosophical investigations. 
Viruses have been virtually absent from philosophy of biology.  In this editorial introduction, we explain why we think viruses are philosophically important.  We focus on six issues (the definition of viruses, the individuality and diachronic identity of a virus, the possibility to classify viruses into species, the question of whether viruses are living, the question of whether viruses are organisms, and finally the biological roles of viruses in ecology and evolution), and we show how they relate to classic questions of philosophy of biology and even general philosophy.