A swine flu outbreak that appears to have caused fatalities in humans in Mexico and nonfatal cases in the United States prompted the World Health Organization (WHO) this weekend to urge countries around the world to be alert for suspicious cases of influenza. WHO Chief Margaret Chan says the global body is taking the outbreak very seriously, though comparisons with the 1918 epidemic are premature. Here are some facts about swine flu : What is swine flu ? Swine flu is a highly contagious acute respiratory disease normally found in pigs. It spreads through tiny particles in the air or by direct contact. According to WHO it tends to infect large numbers of a given pig population, killing between 1 and 4 percent of those affected. Not every animal infected displays symptoms. Where do outbreaks occur ? Swine flu is considered endemic in the United States, and outbreaks in pigs have also been reported elsewhere in North America, South America, Europe, Africa, and parts of eastern Asia. What ...