World Health Organisation head Margaret Chan has warned a forum in Mexico that the swine flu virus worldwide is now unstoppable.
The WHO says most H1N1 cases are mild, with many people recovering unaided.
As the summit opened, the United Kingdom alone was projecting more than 100,000 new cases of H1N1 a day by the end of the summer.
"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.
Turning to the summit venue, the WHO chief added, "Mexico is a safe, as well as a beautiful and warmly gracious place to visit."
Leaders and experts from 50 countries are in Cancun for the two-day meeting to discuss strategies for combating the virus.
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.
