A good post I stole from survivalistboards:
When Doctors refer to a pathogen being airborne, they mean it in a very different way than I think most of us do... They mean it possesses two simple traits. 1. It is very light and can stay suspended in the air, Like DUST but millions of times smaller. And 2. It can survive for long periods of time in that state, or once it lands on something. You can literally catch an Airborne virus by walking into a room where a sick person was an hour before, even if you walk back out without touching anything. Or through the HVAC vents in a building. Breathing infected air is enough.
That's not Ebola... (And hopefully it never will be...)
Ebola is spread through the air via DROPLETS. Meaning a sneeze. A cough. A touch of the nose or mouth and an unwashed hand. etc... A microscopic drop of snot or blood on a table can have thousands and millions of ebola virus in it. The virus can live in that drop for many minutes to a few hours depending on the nature of the fluid and the amount, etc... But will eventually die because its fairly fragile outside the host. Touch that drop, before the virus dies, and you risk getting infected, either through a cut in the skin or a mucous membrane. Compare this to HIV which is neither airborne, nor spread by droplets. You can't get HIV from a cough or a sneeze. You pretty much have to get an "ejection" of body fluids into your body to catch HIV, because simply exposing the virus to air is enough to kill it. And yet 50,000 people get infected with HIV every year.
So what does this all mean for us? Well, it is true many influenza causing viruses are in fact "airborne" in the medical sense. However, research suggests that the vast majority of influenza cases are transmitted via droplets. An uncovered sneeze... An unwashed hand... The same way Ebola is...
Between 5% and 20% of the American population contract influenza every year (despite a vaccine, which we dont yet have for Ebola).
Lets say Ebola runs on the low side of that- 5% That's 17 million infections... And 50-90% of those will be fatal.
20% means 70 million cases... and 40 million+ dead.