A deadly swine flu never seen before has broken out in Mexico, killing at least 16 people and raising fears of a possible pandemic. World Health Organization officials said the flu has killed about 60 Mexicans.

Here are some facts about the virus and flu viruses in general:

* The World Health Organization has confirmed at least some of the cases are a never-before-seen strain influenza A virus, carrying the designation H1N1.

* Although it's called swine flu, this new strain is not infecting pigs and has never been seen in pigs. The threat is person to person transmission.

* It is genetically different from the fully human H1N1 seasonal influenza virus that has been circulating globally for the past few years. The new flu virus contains DNA typical to avian, swine and human viruses, including elements from European and Asian swine viruses.

* The World Health Organization is concerned but says it is too soon to change the threat level warning for a pandemic-- a global epidemic of a new and dangerous flu.

* When a new strain of flu starts infecting people, and when it acquires the ability to pass from person to person, it can spark a pandemic. The last pandemic was in 1968 and killed about a million people.

* Seven people in the United States have been diagnosed with the new strain. All have recovered but the U.S. Centers for Disease Control and Prevention expects more cases.

* Flu viruses mutate constantly, which is why the flu vaccine is changed every year, and they can swap DNA in a process called reassortment. Most animals can get flu but viruses rarely pass from one species to another.

* From December 2005 through February 2009, 12 cases of human infection with swine influenza were confirmed. All but one person had contact with pigs. There was no evidence of human-to-human transmission in those.

* Symptoms of swine flu in people are similar to those of seasonal influenza -- sudden onset of fever, coughing, muscle aches and extreme tiredness. Swine flu appears to cause more diarrhea and vomiting than normal flu.

* Seasonal flu kills between 250,000 and 500,000 people globally in an average year.

* In 1976 a new strain of swine flu started infecting people and worried U.S. health officials started widespread vaccination. More than 40 million people were vaccinated. But several cases of Guillain-Barre syndrome, a severe and sometime fatal condition that can be linked to come vaccines, caused the U.S. government to stop the program. The incident led to widespread distrust of vaccines in general.

Mexico Medicine