A man wears a face mask as a precaution against swine flu while attending an outdoor mass in front of the Basilica of Guadeloupe, the interior of which is closed to the public to contain the swine flu outbreak, in Mexico City

Mexico prepared for the shutdown on Friday of all but essential services to help combat the spread of swine flu as a major US food producer rejected speculation that one of its pig farms in the country was the origin of the disease.