It isn't a question of if but of when.
Spanish Flu example
The pandemic lasted from June 1918 to December 1920, spreading even to the Arctic and remote Pacific islands. Between 50 and 100 million died, making it one of the deadliest natural disasters in human history. Even using the lower estimate of 50 million people, 3% of the world's population (1.86 billion at the time) died of the disease. Some 500 million, or 27% (≈1/4), were infected.
If 3% of the population died in today's world of 7 billion, that would be 210 million killed and 1.89 billion infected.
Note that no part of the world was unaffected.
There are a few things that will make it worse this time around - more international travel, today's modern communication and 24hr cable fear mongers, just in time inventories and the breakdown of the family will make things worse this time.