Speaking at a flu summit in Mexico, WHO's Margaret Chan said most influenza A H1N1 cases are mild and people recover without treatment.
As the summit began in Cancun, British health officials predicted more than 100,000 new cases of swine flu a day by the end of the summer, the BBC reported.
Health officials and experts from 50 countries came to the two-day meeting to discuss strategies to fight the virus.
Chan said pregnant women and people with underlying health problems were at higher risk from swine flu complications than others and should be monitored.
The virus has infected more than 70,000 people in more than 100 countries and killed more than 300, the BBC said.
The United States has had the most confirmed cases since the outbreak in April, with 33,902 confirmed cases and 170 deaths, WHO said.