How the swine flu outbreak is unfolding: a timeline

In early March 2009, people began falling sick in alarming numbers in parts of Mexico. Tests would later confirm that they were the first victims of a new strain of swine flu virus.

Within three months, the outbreak had spread enough for the World Health Organization to declare the world's first flu pandemic in 41 years.The timeline belowillustratessome key dates in the evolution of the first pandemic of the 21st century.