human extinction by a virus

We certainly can’t prove it to be impossible.

However, when we look at the history of disease, there hasn’t been anything that could accomplish an extinction-level outcome. Not even close. Even some of the diseases that we’re most afraid of only kill a percentage of victims. The black death only killed perhaps 25-50% of those infected. Diseases with higher kill rates tend to top out in the 90-percents. Killing 99% of humans is bad… very, very bad… but it still leaves 70 million survivors. In other words, we’d still be one of the most numerous species on the planet for our size.

Even if you engineer the disease, you’re just looking at such a diverse range of genetics, diets, habits, environments, etc. We’ve got people living with minimal human contact out in the wilds of Alaska, people who live on sail boats, people with household bomb shelters… I just don’t see anything getting all of them. Collapse of civilization, certainly, but not extinction.

Not necessarily:

http://www.bt.cdc.gov/agent/smallpox/vaccination/facts.asp