Viruses present elegant systems for the study of evolution in real time—RNA viruses especially so as their replication mechanism is error prone. This essentially results in each RNA genome synthesized in an infected cell having one or more mutations, making the new RNA progeny a population of mutants. Such random mutability offers RNA viruses the chance to adapt quickly to new hosts. This mini-course offers a basic introduction to what viruses are, how they are constructed and how mutations happen. Gain a better understanding of the concept of “emerging viral diseases” and how viral pandemics can occur. Examples of past pandemics will be discussed in this context.