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H1N1 influenza, otherwise known as "swine flu," is a respiratory infection of pigs caused by a certain type of flu -- Influenza A. This regularly causes outbreaks of flu among pigs.

Normally, humans don't get it. But if they do, it usually is the result of direct contact with live pigs. Humans cannot get H1N1 flu from eating pork or pork products.

Human-to-human infections do occur and have been well documented by the Centers for Disease Control. This same strain has killed five people in the United States and and more than 60 people in Mexico.

"There is influenza in humans. There is influenza in swine. There is influenza in birds. Swine flu goes pig to pig to pig, and occasionally it goes out to humans. This one has jumped to humans and it's jumped from human to human - it keeps jumping," said Dr. Martin Blaser of NYU Langone Medical Center. "Avian flu is bird, bird, bird. Occasionally it is humans but it hasn't jumped much further. This strain has the capability of going from human to human. That's what makes it more dangerous."

It is still important to be vigilant about prevention because the flu virus can still easily spread.

The people experiencing severe symptoms, such as difficulty breathing, fever, sore throats and other flu-like symptoms should seek out healthcare and treatment if needed.

For serious cases, H1N1 flu is treated the same as regular flu, with Tamiflu and Relenza. Mild cases are taken care of with plenty of bed rest, fluids and fever medication.

"For the general public. When you cough, cover your mouth. That is the main way influenza spreads," says outgoing New York City Health Commissioner Thomas Frieden. "If you're sick, stay home. If you've got a fever and a cough, stay home. That helps you get better and it prevents spreading the disease to others."

While five people in the United States have already died from H1N1 flu, regular seasonal flu kills about 40,000 people in the United States every year.

Influenza is usually most lethal among the very young, the very old or those with pre-existing conditions and compromised immune systems.