IT HAS BEEN just a few days since the world began learning about a new strain of quickly spreading swine flu.

Although none of the 40 known flu cases in the United States has led to death or serious injury, that hasn't stopped the nation's public-health machinery - and, of course, the news media - from quickly shifting into hyperdrive.

President Obama said yesterday that the swine-flu outbreak - with origins in central Mexico - is a cause for "concern" but not "alarm," but how worried should you be?

The Daily News tries to break it all down for you.

Q. Seriously, why all the fuss?

A. Human outbreaks of swine flu - an illness caused by types of viruses that are typically endemic to pigs (hence the name) - can occur, but are typically rare. The last time there was major public concern over swine flu in the United States was in 1976, when a soldier at Fort Dix died and millions of Americans were immunized as a precaution.

The current outbreak of swine flu - involving a strain of the virus known as H1N1 - has alarmed public-health officials since it was confirmed in Mexico, with isolated U.S. cases, in late March. That's because it seems to spread quickly among humans, its symptoms develop quickly, and in Mexico the illness is killing previously healthy adults, primarily aged 25 to 45.

Q. What is the impact so far?

A. As of last night, there were 149 confirmed deaths in Mexico, and at least 40 confirmed cases in the United States. None of these were fatal, and 28 were reported in a cluster in one New York City prep school.

Q. Any cases in Philadelphia?

A. No. In fact, there have been no officially reported cases in Pennsylvania. Last night, however, health officials in New Jersey said that they've identified five probable cases - none serious, all among people who recently traveled to Mexico or California.

Q. I haven't been to Mexico, so should I not be too worried?

A. Right now, the bulk of U.S. cases appear to be related to travel to Mexico or southern-border states, but that could change quickly given the apparent speed with which the flu spreads.

For that reason, you should know the symptoms, which are fever, cough, sore throat, body aches, headache, chills, fatigue, and occasionally diarrhea and vomiting.

Q. What are Philadelphia officials doing?

A. Yesterday afternoon, a meeting was conducted among Philadelphia health officials, led by Dr. Caroline Johnson, director of disease control. The purpose was to update her staff on the flu and to organize a city response to it. For now, city officials are monitoring the situation.

Q. I have a long-planned vacation to Mexico. Should I still go?

A. Perhaps not. Yesterday, U.S. health officials advised canceling what it called "nonessential travel" to Mexico - an advisory, they added, that they were issuing out of "an abundance of caution."

Most airlines are allowing customers booked for Mexico to change their tickets without a penalty. Experts say travel insurance may cover a cancellation if you bought the policy before April 24, the date the insurance companies say the outbreak became a "known event," but check the fine print.

Meanwhile, the European Union advised its residents against traveling to the United States, because of the rising number of American cases. It appears that global travel restrictions will increase after the World Health Organization raised the threat level to a 4 (out of 6), which means that the disease is spreading quickly among humans.

Q. How do humans get the flu from pigs, anyway?

A. Good question - one thing that health officials are looking at is the role of so-called "factory farming" of pigs, which has frequently been assailed by activists for unsanitary conditions.

Reports in the Mexican news media have suggested that the outbreak traces to the town of Perote in that nation's Vera Cruz province, where a subsidary of U.S.-based Smithfield Farms has a massive hog operation. The American food processor is denying complaints from local residents that the breeding grounds for the disease were the large lagoons of pig manure associated with the operation in Perote.

Q. What else can I do?

A. Always cover your mouth if you cough and wash your hands afterward. Those are still the best way to prevent the spread of any airborne virus. Doctors say that the swine flu can be treated with medicines such as Tamiflu, but they don't advise anyone to take these types of medications unless an infection is confirmed. *

Staff writer Valerie Russ contributed to this report.