Watching viruses form, one virus at a time*

Simple viruses consist of RNA and proteins that form an icosahedral shell (called a capsid) that protects the RNA. Many simple viruses are self-assembled: mixing the RNA and the capsid proteins in a test tube leads to spontaneous formation of infectious viruses in high yield. This result suggests that we can understand RNA virus self-assembly from the perspective of statistical physics. The central question is how a random process like self-assembly can lead to a high yield of well-formed viruses. I will discuss some potential answers to that question based on experiments that measure the kinetics of assembly of individual viruses.