The 2009 swine flu outbreak is the spread of a new strain of H1N1 influenza virus that was first detected by public health agencies in March 2009. Localized outbreaks of influenza-like illness (ILI) were detected in three areas initially in Mexico and soon after in the United States and Canada. Following the discovery of the new strain in the United States, its presence was quickly suspected in several continents, with over 1,600 candidate cases by April 27. Because it is not possible to confirm every one of such cases as being caused by an influenza virus, the World Health Organization (WHO) refers to them collectively as influenza-like illnesses (ILI).