Jason Gale Bloomberg April 24, 2009 Disease trackers are trying to determine whether a previously unseen strain of influenza in the U.S. is related to more than 130 cases of severe respiratory illness in Mexico and may spark a pandemic. A new variant of H1N1 swine influenza has sickened at least seven patients in California and Texas, the Atlanta-based U.S. Centers for Disease Control and Prevention said yesterday. Mexicoâ€™s Health Minister Jose Cordova canceled classes in the capital, Mexico City, today and recommended citizens avoid public places after 20 fatalities from an illness possibly caused by a flu virus. Scientists in both countries and Canada are studying the illnesses to determine whether they pose a larger public health threat. A pandemic can start when a novel influenza type-A virus, including swine flu, begins spreading, since no one has a natural immunity. The so-called 1918 Spanish Flu pandemic, which may have killed as many as 50 million people, began when an avian flu virus jumped to people, experts said.