The whole world is talking about swine flu as scientists scramble to learn more about this emerging disease. Outbreaks of this infectious new strain of influenza virus have proved deadly in some places, relatively benign in others. Some people are avoiding travel to infected areas. Others wonder about a possible vaccine.

Influenza has been with us for a long time. In ancient Greece, Hippocrates described it 2,500 years ago.

In the 20th century, a worldwide epidemic - a pandemic - known as the Spanish flu killed tens of millions, though the same virus today might not be as deadly because medical care is so much better.

Another flu pandemic emerged in 1957. It was known as the Asian flu and killed an estimated one million people. A third but less serious pandemic, the Hong Kong flu, broke out in 1968.

Swine flu first came to widespread attention in 1976 after an outbreak in the United States. Health officials ordered mass vaccinations, fearing a repeat of the 1918 pandemic, but the disease never reached that level.

The new swine flu virus is actually a genetic mashup, containing bits of human and bird flu, as well as a variety that infects pigs.

"Well, in some ways the term 'swine flu' is a misnomer," says Dr. Gregory Gray, who heads the Center for Emerging Infectious Diseases at the University of Iowa.

"This particular strain of influenza may have components of swine influenza," Gray adds, "but it's acting more like a traditional annual influenza that we see every year."

Mexico seems to be the place where this swine flu first infected large numbers of people. Dr. Joan Nichols, an influenza researcher at the University of Texas and the Galveston National Laboratory, says the new viral strain found the right conditions to emerge.

"In this case, if it's a swine virus, it came out of a population where you had a lot of rural communities, pig farms in close proximity to either wild birds or domestic bird populations to get the avian mix into it," Nichols says. "Anywhere you have animals and agricultural settings, you have people in close proximity working with them. So in terms of the agricultural side, I mean, that's the side that would allow for transmission."

Reflecting heightened concerns that international outbreaks of the swine flu could accelerate, the World Health Organization has increased its global alert status, and some governments are discouraging travel to affected areas. But Dr. Keiji Fukuda, the WHO's assistant director-general, said on Monday that there is little point in imposing travel bans "predominantly because this virus has already spread quite far and at this time, containment is not a feasible operation."

Experts say the influenza can be spread from person to person in several ways - by droplets in the air from sneezing without covering your mouth and nose, or from kissing, or from touching contaminated surfaces. But despite the name, they say you can't get swine flu from eating properly cooked pork products.

So far the disease has been much more severe in Mexico than in any other country - more cases there, and more serious ones. The acting head of the CDC - the U.S. Centers for Disease Control and Prevention - says the reason for that is still unclear.

"That is a critical question," says Dr. Richard Besser. "What we need to understand is why we're seeing a different disease spectrum in Mexico than we're seeing here. There are many reasons that could explain that, and as we gather information, we hope to sort that out. As we continue to look, I expect that we will see additional cases, and I expect that the spectrum of disease will expand."

Swine flu can only be identified definitively by laboratory tests. Dr. Joe Bresee of CDC's Influenza Division says swine flu symptoms are not very distinctive.

"Symptoms are similar to the symptoms of regular human flu and can include fever, cough, sore throat, body aches, headache, chills or fatigue. Some people with swine flu have also reported diarrhea or vomiting. Like seasonal flu, swine flu may cause a worsening of underlying chronic medical conditions."

That's from one of the informative podcasts CDC is producing to get out information about swine flu.

The CDC is laying the groundwork for the possible production of a swine flu vaccine, though it would be months before large quantities could be produced. The annual flu vaccine may provide some protection, and Joan Nichols of the University of Texas says antiviral drugs may also be helpful.

"We're also lucky in this case that the virus is sensitive to some of the drugs that we use to treat influenza. And that's a really good thing. That drug is ready immediately, and a number of countries, including the U.S., have stockpiled these drugs."

Whatever the situation now, it's likely to change in the coming days, weeks and months as scientists and public health officials continue to monitor this emerging disease . They're hopeful that it will run its course quickly, but mindful, too, of influenza's notorious past.