Spread of Swine Flu 'Unstoppable': WHO

World Health Organization chief Margaret Chan told a summit in Mexico today that, though most cases of swine flu were not deadly, the virus cannot be stopped, the BBC reports. “With well over 100 countries reporting cases, once a fully fit pandemic virus emerges, its further international spread is unstoppable,” Chan said—adding that public perception is also tough to control.

Chan told the gathering of international experts that most cases of the H1N1 virus—which has killed 300 worldwide—resolved themselves without treatment, but that pregnant women and the infirm were at risk. “For a pandemic of moderate severity,” she said, “this is one of our greatest challenges: helping people to understand when they do not need to worry, and when they do need to seek urgent care.” (More Margaret Chan stories.)