The outbreak is only about 10 days old, and even if the illness is declining, it could return, said Gregory Hartl, the WHO spokesman for epidemic and pandemic diseases, at a briefing Sunday.
"I ... would like to remind people that in 1918 the Spanish flu showed a surge in the spring, and then disappeared in the summer months, only to return in the autumn of 1918 with a vengeance," Hartl said. "And we know that that eventually killed 40 million to 50 million people."
Mexican authorities believe the virus's most active period in Mexico was between April 23 and April 28, and Mexican Health Secretary Jose Cordova described the outbreak as being in decline in his country.
As of Sunday, WHO confirmed 898 cases of swine flu -- known to scientists as influenza A (H1N1) -- reported in 18 countries.
Our prayers go out to the families affected by this pandemic.