The assassination of Franz Ferdinand has a few points to it, the first of which is that it did start the first world war, after which, the allies left germany a mess, in a huge depression, worse by far than that suffered in the united states, which is how Hitler came to power in the first place. Thus, one event indirectly caused two world wars. In addition, technology advances much faster in a state of war than it does in a state of peace. Without the two world wars, we would likely be in a technological state similar to the 60'* or 70'* at this point. Despite this, I think the answer is more the invention of rapid, safe air travel more than anything else. The invention of rapid air transit is the invention that puts the human race back in the state where an outbreak of disease improperly handled could cause near-obliteration of our species.
1994 Infiniti Q45: Grey metallic exterior, light grey interior, 132k