Passengers arriving from Mexico wear masks at Frankfurt's airport April 28, 2009. The World Health Organization has raised its alert level to indicate a significantly increased risk of pandemic, or global outbreak of serious illness. The WHO said the swine flu was being spread by human-to-human transmission but it did not advise any travel restrictions or border closures. REUTERS/Johannes Eisele (GERMANY HEALTH TRAVEL)
REUTERS/JOHANNES EISELE