New Technology

The World Health Organization cautioned that the swine flu outbreak could gain momentum in the months ahead, despite claims by the health secretary of Mexico -- the epicenter of the outbreak -- that the virus "is in its declining phase."

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.

Mexico has 506 confirmed cases, including 19 deaths, WHO reported.