The internet is becoming one global consciousness. As a result, there can never be an authoritarian global government - any attempt will fail. Communication across the globe is too easy.
A fast spreading killer disease can pop up any time. It's just as likely in 20 years as it was 20 years ago.
If anything, it will be famine. We are overpopulating the world - but we *must* have room for farmland. More population = more necessary farmland. Unfortunately, this doesn't work - since they both take space and the earth is finite.
From fire to antibiotics, human beings have always found a way. Our most important discoveries come in times of need.