WASHINGTON (Reuters) - A new and unusual strain of swine flu is likely widespread and impossible to contain at this point, experts agree.
The H1N1 strain has killed at least 20 people and possibly 48 more in Mexico and has been confirmed in at least eight people in the United States, all of whom had mild illness.
Probable cases also were found at a school in the New York City borough of Queens and experts at the U.S. Centers for Disease Control and Prevention say they fully expect to find more cases. Here is why:
* This new strain of influenza has shown it can spread easily from person to person.
* It has been found in several places and among people who had no known contact. This suggests there is an unseen chain of infection and that the virus has been spreading quietly.
* This can happen because respiratory illnesses are very common and doctors rarely test patients for flu. People could have had the swine virus and never known it.
* At least in the United States, it has so far only been found in people who had mild illness, another factor that would have allowed it to spread undetected.
* World Health Organization director Dr. Margaret Chan has said the new strain of H1N1 has the potential to become a pandemic strain because it does spread easily and does cause serious disease.
* CDC experts note that while it is possible to contain an outbreak of disease that is in one limited area, once it is reported in widespread locations, the spread is impossible to control.
Editing by Xavier Briand