In your opinion, what could be the possible reasons of a sudden outbreak of new disease? What are the ways of addressing a new and virulent contagious disease?

In your opinion, what could be the possible reasons of a sudden outbreak of new disease?
What are the ways of addressing a new and virulent contagious disease?
Discuss in detail about the countries, which have faced epidemics in the past. What would have been the possible reasons for such outbreaks?