JUDY WOODRUFF: First, the flu outbreak. The U.S. numbers continued a slow increase, with 155 cases in at least 19 states. And Mexico City was largely closed in a bid to halt the spread there. Margaret Warner has the story.
MARGARET WARNER: The Mexican capital, one of the world's largest cities, lay quiet, as a five-day shutdown began, affecting everything but essential government services.
To the south, another flu victim was buried in a remote Oaxacan village, but there were no new deaths in Mexico overnight.
And a top Mexican health official charged the World Health Organization might have contained the outbreak earlier.
The country's chief epidemiologist said he reported the disease to the WHO's regional office on April 16th, a week before the organization took action.
In Geneva, a WHO spokesman disputed the claim.
GREGORY HARTL, World Health Organization: It was not until the 23rd of April that we received confirmation from U.S. CDC in Atlanta that we were dealing with the same virus north and south of the border.
And it became very clear at that time that this was a very unusual event. And from that moment on, we activated our 24/7 operations center. The following day, we declared a public health emergency.
MARGARET WARNER: By today, the virus, known scientifically as H1N1, had spread to at least 13 countries. Hong Kong, China, was added to the list today, the first known case in Asia. Officials sealed off a hotel where a 25-year-old Mexican tourist with the flu had stayed.
And in Germany, where a handful of cases have appeared, officials confirmed the first human-to-human transmission there.
In the U.S., there were some encouraging words. President Obama urged calm again, and he said the U.S. government is prepared.
PRESIDENT BARACK OBAMA: I'm optimistic that we're going to be able to manage this effectively, but we still have more work to do. And I'm glad I've got such a great team doing it.
It may turn out that H1N1 runs its course like ordinary flus, in which case we will have prepared and we won't need all these preparations.
MARGARET WARNER: And the Centers for Disease Control in Atlanta announced this flu strain does not have the deadly genetic mix of the 1918 pandemic that killed tens of millions.
CDC flu chief Dr. Nancy Cox.
DR. NANCY COX, U.S. Centers for Disease Control: We do not see the markers for virulence that were seen in the 1918 virus. However, we know that there's a great deal that we do not yet understand about the virulence of the 1918 virus or other influenza viruses that have a more severe clinical picture in humans.
MARGARET WARNER: The effort to get a vaccine into production was also underway, but the World Health Organization said again it will be a matter of several months.
In the meantime, 250,000 children were out of school in the U.S. as more schools and districts shut down as a precaution. More people have filled prescriptions for antivirals this week, too.
The research group SDI reported prescriptions jumped nine-fold and surgical masks flew off the shelves.
WILLIAM BROWN, Winter Park Drug Store: I was surprised, just because I'm not sure how effective they are at preventing someone from catching a virus.
MARGARET WARNER: Public fears were also evident in a new survey from the Harvard School of Public Health. Of more than 1,000 people, 46 percent said they were concerned or somewhat concerned that they or a family member would get this flu in the next 12 months.
In addition, one in four said they're avoiding crowded places like malls or public transportation. And about 60 percent said they're washing hands and using hand sanitizer more often.
There were also new questions today about exactly where the disease originated. Reports pointed to two children in California, reporting symptoms days before the earliest known Mexican case.