Swine Flu Map

The cases in the U.S., like those in Canada, have been milder than Mexico, but "as we continue to look for cases, we expect to see more cases in a broader spectrum — and that is what we are seeing," said Dr. Besser.

View 2009 Swine Flu (H1N1) Outbreak Map in a larger map

The disease originated in pigs. But health authorities in the U.S. are going to start referring to it as the 2009 H1N1 virus, rather than swine flu, to halt any misconception that it can be spread by eating pork.

Ordinary influenza normally claims 36,000 lives in the United States, which prompted reporters to ask why there has been so much attention paid to this particular virus.

American health officials explained that it is a virus that has never been seen before, which means there is no background immunity in the population, and it is spreading from human to human. That means it has the potential to become a pandemic.

"At the start of an outbreak, you don't know what the course is going to look like," Dr. Besser said. "You don't know if this is a virus that is going to fizzle out in a couple of weeks or one that is going to become more or less severe in the diseases it causes."

The American border will remain open. But, as in Canada, customs officials will be actively looking of people who are sick.

With reports from Karen Howlett, The Associated Press and The Canadian Press