||P.1 left column: "RNA viruses evolve extremely rapidly, often with mutation rates one million times greater than vertebrate species [primary source]. This rate of mutation allows viral populations to keep pace with rapidly changing environments. Viral pathogens, such as influenza virus, HIV, hepatitis C virus and measles virus, place a substantial burden on global human health. Often, after encountering a particular viral strain, an individual develops long-lasting immunity specific to this strain. However, in some viruses, mutations to the virus genome may result in proteins that are recognized to a lesser degree by the human immune system, leaving individuals susceptible to future infection. These mutations rapidly spread through the virus population in a process known as antigenic drift. The capacity for rapid evolutionary change allows the virus population to flourish, despite substantial immune pressure."