Super bugs like Ebola often wipe out a village so fast that few survivors can travel to infect other areas. The worst are bugs that kill about 10% and have a long dormant period where the victim can move about and transmit to others.
That's my take on it. The most successful bugs, obviously, are those that kill a very small percentage and then can remain in general circulation until the next mutation -- like our present flu.
The thought that there might be a moderately successful bug that kills 10% is frightening -- but let's be perfectly frank here. That still gives everyone tremendous odds for survival. I'll take a 90/10 any day of the week.
Nonetheless, a literal decimation of the global population would put us in a 10-15 year depression that would make the 1929 crash look like party-time.