The 2013 flu season is one of the worst in recent history.

America is engulfed in a sea of influenza, an especially potent viral strain that's sickening people from coast to coast. And America's doctors are making no qualms about it—it's a nasty flu season—one of the worst in years. Data from the U.S. Centers for Disease Control and Prevention shows the flu is still elevated across the country, with 24 states and New York City reporting high rates of doctor's visits by people trying to get some relief from the virus. This year's flu strain, H3N2, is known to be particularly aggressive, churning out more severe symptoms and leveling people for up to two weeks.

The 2013 flu is serious. Here's what you need to know:

#1. We're in the thick of it.
The 2012–13 flu began unseasonably early, but that doesn't mean it's going to make a much-hoped-for early exit. Areas in the South first hit by the flu are still seeing steady rates of hospitalization for complications, and the West Coast has just seen the first week of its seasonal onslaught. "We're not over the hump yet," says William Schaffner, MD, an infectious disease expert with the Vanderbilt University Medical Center. "Basically, we're in the middle of it."

#2. Make the call.
Dr. Schaffner urges people to call or visit their doctors when they first start showing telltale flu symptoms: Fever, feeling feverish or experiencing chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, fatigue, and sometimes vomiting and diarrhea, though this is more common in children. " Although flu is principally a respiratory disease, it has an impact throughout the entire body," he explains.

If your doctor IDs the flu within the first day or two, he or she may be able to prescribe an antiviral drug that can shorten the duration of the infection and reduce the risk of complications. That said, Dr. Schaffner says you should always see a doctor if your symptoms include a very high temperature, coughing up sputum, or difficulty breathing, or if a baby or young child becomes extremely lethargic. Doctors can then test for complications including pneumonia.

#3. Know thy enemy.
There's a gang of viruses patrolling the streets this winter, some of them easily mistaken for the flu. Norovirus targets your intestinal tract much like the flu, but remember, the flu packs a whole-body punch, including body aches and respiratory symptoms. Colds are also circulating, but generally produce "above-the-neck" symptoms.

Read More: Is it the Flu or Something Else?

#4. It's not too late to boost your immune system.
Dr. Schaffner still urges people to get the flu shot because the virus is likely going to linger into March. Whether you get a flu shot or not, it's always a good idea to bolster your immune system. When you're feeling well, exercise and even sex produce immune-boosting benefits. Ask your doctor to test your vitamin D levels, too. If they're low, ask for supplement recommendations to help bring your levels into the normal zone where they can help support your immune system. And of course, keep your body healthy by eating a diet rich in vitamin C and avoiding processed foods whenever possible.

Read More: 25 Foods That Fight Cold & Flu

#5. Note the new flu shot in town.
Getting a flu shot is a personal choice, although most healthcare experts recommend it. But now there's a new vaccine in town. You know that flu vaccine you're hearing about? The one that's made using an insect virus? About 150,000 doses of FluBlok are expected to hit the market at the tail end of the 2013 flu season. The Food and Drug Administration (FDA) approved the vaccine for use for 18- to 49-year-olds, saying it is different than other flu vaccines on the market because it isn't grown using eggs. This allows for much faster manufacturing. The approval came after a clinical trial of 2,300 people found FluBlok prevented the flu in about 45 percent of cases and was effective against all circulating flu viruses, not just the strains included in the vaccine.

According to Protein Sciences Corporation, the Connecticut-based manufacturer, Flublok does not contain the controversial mercury-containing preservative thimerosal. "This approval represents a technological advance in the manufacturing of an influenza vaccine," says Karen Midthun, MD, director of the FDA's Center for Biologics Evaluation and Research. "The new technology offers the potential for faster startup of the vaccine manufacturing process in the event of a pandemic, because it is not dependent on an egg supply or on availability of the influenza virus."

Health Decoder: The Facts About Flu Shots!