The new strain was identified as a combination of several different strains of Influenzavirus A, subtype H1N1, including separate strains of this subtype circulating in humans (see human influenza) and in pigs (see swine influenza). The strain transmits between humans and was initially reported to have a relatively high mortality rate in Mexico. The World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) have expressed serious concerns that the new strain has the potential to become an influenza pandemic.[146] It is reported that, because the virus is already widespread, containment will be impossible.[147]