The 12 deadliest viruses on Earth

There are viruses that are deadly, and some that are even deadlier. Some viruses, including the novel coronavirus currently driving outbreaks around the globe, have lower fatality rates, but still pose a serious threat to public health as we don't yet have the means to combat them.
Here are the 12 worst killers, based on the likelihood that a person will die if they are infected with one of them, the sheer numbers of people they have killed, and whether they represent a growing threat.