How can a disease transition from a basic illness to a flown blown pandemic

Pandemics occur for two reasons. The first is a change in the infectious agent and the second is in human patterns. Both involve exposing people to agents they don't have resistance to. Diseases like SARS, avian flu, and the famous Spanish flu all involved a mutation in the virus that resulted in a new disease that no one had resistance too. Examples of the second are when new groups of humans make contact, like small pox amongst native Americans, and possibly the black death.