The current 2009 H1N1 flu pandemic, or “swine flu” as it was called early on, is caused by a novel strain of influenza A virus subtype H1N1. This new type of virus was first detected in people in the United States in April 2009, and has spread from person-to-person worldwide, probably in much the same way that regular seasonal influenza viruses spread.
On June 11, 2009, the World Health Organization (WHO) declared the H1N1 outbreak to be pandemic;
On August 28, 2009, the WHO issued a statement urging people in the northern hemisphere to be prepared for the second wave of the H1N1 pandemic as autumn approaches.