Why Do Viruses Often Seem to Emerge from Nowhere to Become

Why do viruses often seem to emerge from nowhere to become major public health scares in Canada?

A) because viruses are constantly evolving and sometimes this process leads to them gaining access to a new species (e.g., humans)
B) because over time our immune systems are becoming less robust so we are becoming more susceptible to the same viruses
C) because we hear about them on the news, whereas in the past we may have remained unaware of them
D) because the majority of Canadians have stopped getting vaccinated against common viruses