The outbreak of influenza A (H1N1) virus is an epidemic of a new strain of influenza virus identified in April, commonly referred to as swine flu. Though the source of the outbreak in humans is still unknown, cases were first discovered in the US and officials soon suspected a link between those incidents and an earlier outbreak of late-season flu cases in Mexico. Within days, hundreds of suspected cases, many of them fatal, were discovered in Mexico, with yet more cases found in the US and several other countries in the northern hemisphere. Soon thereafter, the World Health Organisation and the US Centers for Disease Control and Prevention (CDC) expressed concern that the A(H1N1) could become a worldwide flu pandemic, and WHO raised its pandemic alert level to Phase 5 out of the six maximum, as a signal that a pandemic is imminent. Although virologists have noted that the outbreak has proven relatively mild and less fatal than historic pandemics, other health officials worry that there are no signs of this petering out.