The current Ebola outbreak that began in March 2014 is the largest in history. The three most affected countries are Guinea, Liberia and Sierra Leone, and cases have also been reported in Nigeria, Senegal, Spain and the United States. Live Science is covering the outbreak; check back for updates on the outbreak.
Humans have been fighting viruses throughout history. Here are the nine viruses that are the world's worst killers, based on their mortality rates, or the sheer numbers of people they have killed.