Swine influenza, or flu, is a contagious respiratory disease that affects pigs. When the flu spreads person to person, instead of from animals to humans, it can continue to mutate, making it harder to treat or fight, because people have no natural immunity.
Symptoms include fever, runny nose, sore throat, nausea, vomiting and diarrhea.
The WHO's "Phases of Pandemic Alert," which has been in existence for five years, characterizes phase 5 as a human-to-human spread of the virus into at least two countries in one WHO region, signaling that a pandemic is imminent.
The highest level, phase 6, is defined by community-level outbreaks in at least one other country in a different WHO region, according to the agency.
Take a look at this swine flu cases map:
2009 Swine flu cases
