Noble prize winning virologist Joshua Lederberg identified viruses as being a real threat to our existence in his quote:

“the single biggest threat to man’s continued dominance on this planet is the virus”

History suggests Lederberg was correct because there have been numerous examples of epidemics and pandemics in past centuries which have changed human history, for example epidemics of smallpox and measles in the 1500's contributed to the decline of the Aztecs. Famously the Spanish flu pandemic of 1918-1919 killed between 40 and 50 million and infected more than a third of the world's population, in fact WHO note that this pandemic alone killed more people in less time than any other disease before or since.

The most important emergence in recent times was undoubtedly HIV/AIDS virus which caused the largest behavioral-type pandemic in human history. The emergence of the SARS coronavirus in China highlighted two facets of globalization - how a new infectious disease which emerged in a remote area was able to spread across the planet causing 8,000 cases and 900 deaths in 30 different countries but it also demonstrated how scientific co-operation was able to control the disease spread within 8 months. The continuing spread of dengue viruses, the introduction and establishment of viruses such as West Nile, Zika and Chikungunya into new territories highlights the relative ease with which viruses can move in an age of environmental change and cheap air travel. The re-emergence of measles, a virus considered to be controlled by vaccination has highlighted holes in our control measures. Of course the recent re-emergence of Ebola in Africa has highlighted that a virus that has a high fatality rate but low infectivity can affect the world economy.

Of course scientifically and culturally we have evolved significantly since 1918, we now have vaccines and anti-viral drugs, we have improved living conditions for billions of people, more people than ever before have access to clean water, good living conditions and good health care (at least in the first world). We could be forgiven for assuming that we had infectious diseases under control, yet in the last 30 years we appear to have found ourselves in an age of emerging plagues largely of a zoonotic origin. Dealing with this emergent threat will require more than science, it will require an greater understanding of how we affect our environment, our food supply, farming practices, urbanisation, even travelling for recreation because in the 21st century we are only ever one plane ride away from an outbreak.

"The future of microbes and mankind will probably unfold as episodes of a suspense thriller that could be entitled Our wits versus their genes".
J. Lederberg, Science 2000