The 2009 swine flu caused by a new strain of H1N1 was first noticed in April of that year, when health officials detected a virus with a novel genetic makeup. In a few weeks, the virus had spread around the world and triggered a widespread response from health officials. Another version of the same virus was involved in the 1918 flu pandemic that killed 30 million to 50 million people, or 2 percent of the world’s population at the time. [5 Scariest Disease Outbreaks of the Past Century]
As the end of the swine-flu pandemic was announced in 2010, the World Health Organization (WHO) reported the death toll was about 18,500, but the organization warned that the figure reflected only the minority of cases that were confirmed by lab testing.