At last count by the World Health Organization (WHO), the number of confirmed human cases of swine influenza around the world was 40 in the United States, 26 in Mexico, 6 in Canada and one in Spain. The number of cases affecting pigs: zero.

But the situation is dynamic and rapidly evolving, so what’s true today may well be different tomorrow, said Dr. Timothy Uyeki, an epidemiologist with the U.S. Centers for Disease Control and Prevention in a conference call with clinicians Monday.

“There’s a lot that we don’t know about this virus,” Uyeki said.

One important mystery is why the virus is causing severe illness and death in Mexico, while so far causing only mild illness in the United States and Canada. Uyeki said the CDC has sent investigators to Mexico to try to sleuth out answers.

Here is what authorities know at this point:

* The pathogen, an influenza A virus, subtype H1N1, is brand-new, never before having been detected in North America.

* It’s known as “swine flu” because six of its eight genes are similar to genes in existing swine influenza viruses known to be transmissible from pigs to people. The other two genes are distinct. “They’ve never been seen in North America,” Uyeki said, noting that they are most closely related to Eurasian swine influenza viruses.

* None of the confirmed U.S. cases had contact with pigs or swine. In fact, no swine are known to be ill with this flu strain, according to the American Association of Swine Veterinarians.

* Of the 40 confirmed cases in the U.S., patients range in age from 7 to 54, with a median age of 16. A “significant proportion” are under age 18, Uyeki said.

* The cases come from the following five states: New York, 28; California, seven; Kansas, 2; Texas, 2; Ohio, 1.

* Only one of the patients was hospitalized, and only because she had an underlying chronic autoimmune condition.

* By contrast, H1N1 has caused seven confirmed deaths in Mexico, according to WHO’s latest count posted Tuesday. Illustrating the swiftness with which conditions are changing, the Washington Post quoted Mexico Health Secretary Angel Cordova Tuesday as saying that the virus has caused 20 confirmed deaths in his country, with another 149 suspected. All told, Cordova reportedly said, 1,995 people have been hospitalized with swine flu, of which 1,070 have been released.

* The virus is resistant to some antiviral drugs but not all. Specifically, Uyeki said, it does not respond to amantadine and rimantadine but is susceptible to the neuraminadase inhibitors Relenza (zanamivir) and Tamiflu (oseltamivir).

* Symptoms of H1N1 resemble symptoms of common seasonal flu strains: fever, sore throat, cough, runny nose and sometimes diarrhea and vomiting.

* Attempts to develop a vaccine are underway.

Because cases in the United States so far have been mild, it was only by accident that the authorities even discovered the new strain in this country.

Uyeki said people with “uncomplicated flu” symptoms sought medical care at outpatient clinics. “These facilities had surveillance protocols in which specimens were collected with nasal pharyngeal swabs,” he said. “There wasn’t any clinical decision to obtain the specimens. It was simply because the facility was participating in surveillance protocols.”

Although the United States has not seen any severe cases to date, Uyeki cautioned that “We should not expect that we will see only mild cases. ... At the CDC, we are taking the situation very seriously,” he said. “We don't want to send out the message that there will be only mild disease. We simply don't know. We want to be prepared for the potential of serious disease.”

Because the situation fast-evolving, he advised practitioners to check the CDC website frequently for updates: www.cdc.gov/swineflu

Clinicians may send questions to coca@cdc.gov

A transcript of Uyeki's teleconference is online at emergency.cdc.gov/coca