When we look at worlds history, we know that Plague, Cholera, SARS, AIDS, H5N1 and various influenza viruses take so much life and caused a lot of damage. I was playing some Plague Inc it is a game about exactly this topic. Lets discuss the posibility of this doomsday theory.
Biggest Epidemics
There were plently of epidemics humanity faced is the past years. However we are creating a doomsday scenario therefore I want to talk about the biggest one which did the most damage to humanity. The Plague of Justinian and The Black Plague. Plague of Justinian hit humanity between 541 542 AD and 100 million people died just in a year and humanity never faced this kind of catastrophic epidemic in their history untill 1300's. Tragically between 1346 to 1350 Europe lost 60 percent of it's population because of another plague type called Black Plague and approximetely 50 million people died in this epidemic.
What are the Risks ?
You may feel safe because of the current technological development and the strict policies World Health Organization follows however these microscobic creatures evolve pretty fast and get resistent to current threatments. We need to also consider biologically engineered ones for biological warfare. Eventhough we didn't face something like Zombie outbreak or anything realistic this topic is still intriguing subject for a lot of movies. Eventhough we have better equipments to create or cure diseases we also need to consider our development in transportation because if Plague can wipe 50 percent of world population in 1 year. I'm pretty sure something agressive and unique can wipe most of the life in the earth.
Doomsday Scenario
We didn't face any global epidemic for a long time however it is the fear of most of the public health professionals because of the mobility and globalization we are facing in our recent years. If something goes bad in labs it is really easy to imagine a wild rapidly spreading disease wipe out most of our population and because of all those holywood movies lets say it is T virus and we can all become mindless brain eating zombies.
If some new epidemic is going to start it will mosty likely start from animals. because scientist discover new infectious pathogens everyday and when we consider the constant contact between animals the potantial of a wider spread is pretty high. If current healt organizations fails to maintain a strong network between each other and goverments , We will all turn in to mindless brain eating zombies. The best part is we don't have to think anymore just wonder around and eat some brains.
Sources
http://www.worldatlas.com/articles/the-10-worst-epidemics-in-history.html
http://www.hhmi.org/biointeractive/emerging-infections-how-epidemics-arise
