"As we see today, with well over 100 countries reporting cases, once a fully fit pandemic virus emerges, its further international spread is unstoppable," The BBC quoted Dr. Chan, as saying in her opening remarks.
She stressed that the overwhelming majority of patients experienced mild symptoms and made a full recovery within a week, often in the absence of any form of medical treatment.
The exceptions, she said, were pregnant women and people with underlying health problems, who were at higher risk from complications from the virus and should be monitored if they fell ill.
"For a pandemic of moderate severity, 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," Dr Chan said.
It has been more than two months since the initial alert over swine flu. Since then, the H1N1 virus has entered more than 100 countries, infected more than 70,000 people and killed more than 300 worldwide.