With 148 cases of swine flu in nine countries (Mexico has 26, including seven deaths; the U.S. has 91, with one death—14 in California), the World Health Organization raised the influenza pandemic threat level to 5, right under Phase 6, which is a full-blown pandemic. Here's how the WHO defines Phases 5 and 6:

Phase 5 is characterized by human-to-human spread of the virus into at least two countries in one WHO region. While most countries will not be affected at this stage, the declaration of Phase 5 is a strong signal that a pandemic is imminent and that the time to finalize the organization, communication, and implementation of the planned mitigation measures is short.

Phase 6, the pandemic phase, is characterized by community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. Designation of this phase will indicate that a global pandemic is under way.

WHO Director-General Dr. Margaret Chan explained, "This change to a higher phase of alert is a signal to governments, to ministries of health and other ministries, to the pharmaceutical industry and the business community that certain actions should now be undertaken with increased urgency, and at an accelerated pace." She was serious, "The biggest question is this -- how severe will the pandemic be, especially now at the start. It really is all of humanity that is under threat during a pandemic," but also added, "On the positive side, the world is better prepared for an influenza pandemic than at any time in history. Preparedness measures undertaken because of the threat from H5N1 avian influenza were an investment, and we are now benefitting from this investment." The silver lining of the bird flu!

More details at the CDC and WHO. Also, Slate tackled the issue of whether face masks offer protection.