"It's not trumped up. It's not a false alarm. The outbreak is real," he said.

Sundwall and Rolfs said the strain warrants special attention because it is exhibiting behavior traits and strength enough to overcome standard human immunity either provided by vaccines for other types of flu or by natural defenses people build up over time following exposure to previous strains.

Rolfs said the outbreak is coming very late in the usual flu season and seems to be following a similar pattern to the Spanish flu pandemic of 1918. That virus also arrived late in the regular flu season, caused mainly mild cases of infection that spring and summer and then returned mutated and stronger that winter, killing 50 million people worldwide before it subsided and joined the other so-called seasonal flus that cause the 200,000 cases and 36,000 deaths around the world annually.

Key developments on swine flu outbreaks

Deaths: Global total of 87 — 75 in Mexico, 10 in U.S., one in Canada and one in Costa Rica. Officials said victims from Canada, U.S. and Costa Rica also had other medical conditions.

Confirmed cases: WHO says 40 countries have reported more than 9,830 cases, mostly in U.S. and Mexico. That figure does not include Taiwan, which reports its first confirmed case.

CDC says 47 U.S. states and District of Columbia have combined 5,710 confirmed and probable cases. Most probable cases are eventually confirmed.

—Associated Press

E-mail: jthalman@desnews.com