What are causes of pandemics throughout the world?

A pandemic is a global outbreak of influenza and occurs when a new influenza virus emerges, spreads and causes disease worldwide. Influenza A viruses continuously undergo antigenic evolution. There are two main mechanisms by which they do so: antigenic drift causes regular influenza epidemics, while antigenic shift is the cause of occasional global outbreaks of influenza (pandemics). Antigenic shift implies that an influenza A virus with a new HA subtype is introduced into the human population. This may occur by (i) direct transmission of an avian virus from birds to humans, (ii) genetic reassortment between an avian and a human virus, or (iii) reintroduction of an old strain into the population.
A virus may remain hidden and re-emerge as a pandemic virus when the immunity in the population has waned. The virus that caused the Russian flu in 1977 turned out to be identical in all of its genes to the H1N1 virus that caused a major epidemic in 1950.