When it comes to deadly viruses, Ebola kills in the quickest and most horrific way possible— causing massive bleeding and turning internal organs into a soup of lifeless mush. It’s estimated that 90 percent of Ebola-infected people die from these symptoms. However, HIV (the virus that causes AIDS) is still more effective eventually, virtually everyone who contracts HIV will have their immune systems knocked offline, as the virus infects the very T cells that are supposed to defend the body. Without treatment, a person suffering from AIDS is unlikely to last even a few years, as hundreds of ordinarily harmless microbes ravage the body.
However, neither of these viruses can boast the highest body count through history. That dubious distinction probably belongs to influenza, the virus that causes the flu. Each year, influenza kills hundreds of thousands of people across the globe, most often the very old or the very young. But every few generations, a strain appears that is far deadlier, like the 1918 Spanish flu, which killed tens of millions of people in a single, worldwide outbreak.
Source of Information : Oreilly - Your Body Missing Manual