We've been warned for years that we're overdue for a new flu pandemic that could strike the US. It could be a mutated strain of something that's already floating around, or, it could be avian flu passed on to humans. The World Health Organization researchers and U.S. health officials are now closely monitoring a new strain of flu affecting Mexico and the U.S. that has been passed onto humans from swine.

The U.S. Centers for Disease Control and Prevention has said that this particular virus found in the U.S. was a never before seen mixture of viruses typical amongst pigs, birds, and humans. In the U.S. seven patients have been affected, and all have recovered. However, in Mexico it has already sickened more than 600 and killed at least 16 people.

Mexican Health Minister Jose Angel Cordova said Friday:

"It is a virus that mutated from pigs and then at some point was transmitted to humans."

Classes at schools and universities in Mexico City have been suspended in an attempt to contain what might be a new strain of influenza. Cordova said that both private and public schools in the metropolis of 20 million were ordered closed Friday, and could be extended in the coming days.

Hey says that the flu is a "new, different strain that can attack anyone." Cordova also said that Mexican authorities are now investigating whether it is related to the influenza strain that has been reported in Texas and California.

On Thursday the U.S. said that seven people were diagnosed with the new flu in California and Texas. The CDC now believes that it can spread human-to-human, which is unusual for a swine flu virus. They believe that because of the intensive searching now underway, they are likely to uncover additional cases.

CDC officials have detected a virus containing a unique combination of gene segments that have never before been seen in humans or pigs. It contains human virus, avian virus from North American, and pig viruses from North America, Europe, and Asia. While health officials have seen mixes of bird, pig and human virus before, they have never seen such an intercontinental mix with more than one pig virus in the mix.

Scientists consistently keep a close eye on flu viruses emerging from pigs because these animals are considered to be particularly susceptible to avian and human viruses, and are considered a likely place where genetic reassortment can take place that might lead to a new form of pandemic flu.

The CDC says that the virus may be something brand new, or may have been around for some time but was only detected now due to improved lab testing and disease surveillance. It was first detected in the U.S. in southern California in a 9-year-old boy and a 10-year-old girl in neighboring San Diego and Imperial Counties.

Both cases were caught under unusual circumstances. one was seen at a Navy clinic particpating in a specialized disease detection network, and the other was picked up through a specialized surveillance system set up in border communities.

Investigators unveiled that they had discovered five more cases on Thursday, including a father and his teenage daughter in San Diego County, a 41-year-old woman in Imperial County, and two 16-year-old boys that are friends living in Guadalupe County, Texas, which is near San Antonio.

One of the California cases - the 10-year-old boy - had traveled to Texas earlier this month, but to Dallas and not San Antonio. Dallas is about 270 miles northeast of San Antonio. The two 16-year-old Texas boys had not traveled recently.

The swine flu's symptoms are like those of the standard flu - involving mostly fever, cough, and sore throat. Some also experienced vomiting and diarrhea.

The CDC is now beginning to receive samples from Mexico for testing, and US health officials are consulting with Mexican and Canadian health officials.