[cnn-photo-caption image=http://i2.cdn.turner.com/cnn/2009/HEALTH/04/26/swine.flu/art.swine.flu.mex.02.cnn.jpg caption="A girl in Mexico City wears a mask outside the Metropolitan Cathedral on Sunday. "]
No kissing to say hello. No large crowds. No close contact.
That's the advice of the Mexican government as more and more people die of swine flu, which has turned into a "public health emergency of international concern," according to the World Health Organization.
The WHO advised all countries to be on the lookout for "unusual" outbreaks of flu, following an emergency meeting Saturday as the seriousness of the outbreak became clear.
By Sunday, 81 deaths had been deemed "likely linked" to a deadly new strain of the virus by health authorities in Mexico. Viral testing has confirmed 20 cases, said Dr. Jose A. Cordova Villalobos, Mexico's health secretary.
In Mexico City, the massive downtown Cathedral of Mexico City was open but Masses were not scheduled. Dozens of worshippers put on masks and went inside the church anyway to pray on their own.
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