'A World At Risk' report by The Global Preparedness Monitoring Board (GPMB) warns that around 80 million people could be killed by a flu-like illness that's able to spread around the world in around 36 hours.
There are several types of flu viruses including Nipah and Zika with the elderly, hospitalized, or juveniles being most at risk.
The Spanish flu pandemic that happened about 100 years ago in 1918 wiped out 50-100 million people (over 3x more casualties than World War I) and infected about 1/3 of the world. Between 10% and 20% infected succumbed to their illnesses. Imagine how much faster this would've spread once you throw air-travel into the mix. That means the flu viruses could spread in about 36 hours!