The H1N1 flu, formerly known as swine flu, presented mainly mild cases outside the outbreak in Mexico, said Dr. Margaret Chan, WHO director general when addressing the 62nd World Health Assembly in Geneva last week. Mexican health officials blamed H1N1 flu for the deaths of more than 150 people.
The H1N1 virus spreads easily from person to person and rapidly within a country once it is established.
"We expect this pattern to continue," Chan said.
She defended elevating the pandemic threat to Phase 5 on a six-phase scale, saying it activated a number of preparedness measures and saw public health services, laboratories, WHO staff, and industry officials working around the clock.
"A defining characteristic of a pandemic is the almost universal vulnerability of the world's population to infection," Chan said. "Not all people become infected, but nearly all people are at risk."
Health service scientists "are trying to get some answers to a number of questions that will strengthen risk assessment and allow me to issue more precise advice to governments," Chan told the gathering.