A woman wears a mask to prevent from being infected by the swine flu virus in the Mexico City, capital of Mexico, on April 25, 2009. The World Health Organization (WHO) on Saturday declared the swine flu outbreak in Mexico and the United States a "public health emergency of international concern" and urged countries to be alert. [Xinhua]

A woman wearing a mask walks past a poster calling on residents to prevent from being infected by the swine flu virus in the Mexico City, capital of Mexico, on April 25, 2009. [Xinhua]