It is not possible to predict a pandemic in advance, so health officials keep a close watch on viruses that have "pandemic potential" -- new viruses that have shown at least some ability to transmit to humans.
The current H5N1 bird flu circulating in Asia is an example of a virus that has pandemic potential, but has not yet caused a pandemic.
Pandemics can vary quite a bit in severity. The 1918 pandemic killed many more Americans than an ordinary flu, while the 1968 version killed about 34,000 people -- about the same number killed each year by seasonal flu, according to CDC statistics.
The world generally experiences at least two flu pandemics each century.
Declaring a public health emergency allows states to free up their resources for prevention of the disease in question.
That declaration gives the head of the Department of Health and Human Services authority to take rapid measures -- including authorizing contacts and mobilizing the national disaster system -- to respond to the disease, including allowing the use of unapproved drugs.
Scientists have not yet determined the reason why there are reported swine flu deaths in Mexico while cases in the United States have thus far been mild.
The WHO noted that the U.S. cases have all been confirmed by laboratory analysis, while those in Mexico have not all been confirmed as yet. This means that health officials do not know how many people who died in Mexico perished because of the new swine flu virus.
However, the CDC is concerned that because the virus is genetically similar in the United States and Mexico, the United States will see more severe illness as new cases emerge, including some deaths.
What many people do not realize is that the flu is generally a much more serious illness than most people appreciate. Even the "typical" flu season results in 36,000 American deaths each year, according to CDC statistics.
The swine flu is a type of influenza virus usually found in pigs. The most common version is H1N1, and the current virus causing concern is a new variation of that virus. Swine flu does not typically pass to humans directly, but such transmission can occur. The current swine flu virus concerns health experts because it has shown the ability to pass from human to human.
When a person gets the swine flu virus, it takes 48 hours before the infected person actually begins to feel ill.
"It takes time for the virus to get down and start to create an illness. That interval between exposure and onset of illness is called the incubation period. The virus hasn't manigested itself," Schaffner said.
Once a person becomes ill, they can stay that way for anywhere from 48 hours to seven days. So far, in the United States, the cases of swine flu have been mild.
There is no vaccination for the swine flu system. U.S. and WHO officials said they are beginning work on a vaccine, but that could take months to develop.
Two prescription anti-viral drugs -- Tamiflu and Relenza -- have proved effective in combating the swine flu virus in victims in the United States. The Obama administration has released 12.5 million courses of the country's stockpile of 50 million courses of Tamiflu.
Health officials and doctors don't recommend people go to the hospital if they are experiencing flu-like symptoms. Rather, they should contact their doctor.