MERS epidemic was small, so it didn't affect the markets much. Same goes for Ebola - the fact that Bitcoin dropped during that time is a coincidence, as Bitcoin was experiencing a bubble that popped at that time. Now the recent crash was different, because it caused a real panic on stock market, since all countries were taking drastic measures that hurt their economies. Bitcoin was always a risky asset, so this time when things actually got serious, investors started selling it, which triggered a big crash.