One thing is certain as the leaves change and crisp fall days replace the heat of summer: Flu can't be far behind.

The Centers for Disease Control and Prevention's latest advisory says it's impossible to predict what this flu season will be like, but you can count on them to pressure you to get a flu shot anyway — even though picking the right virus to immunize against is a crap shoot.

If you'd rather skip the shot and its risks, you can give your immune system a "shot in the arm" by trying these quick, simple ways to rev up your natural defenses:

Take probiotics. Probiotics are healthy bacteria, such as Lactobacillus and Bifidobacterium, and they help keep your digestive system in tip-top shape. Probiotics can be bought as supplements, but are also found in yogurt.

Numerous studies show that probiotics help the body fight viruses: A meta-analysis of 10 studies found that those who took probiotics reduced their chances of catching a cold by 42 percent.

Sing. Singing, even if it's only in the shower, can unleash immune-fighting antibodies in your blood. German scientists discovered that singing increases the concentration of immunoglobin A, proteins in the immune system that act as antibodies which fight respiratory problems and infections, and hydrocortisone, a hormone which fights stress.

When the blood of people who sang in a choir was tested before and after a rehearsal of Mozart's Requiem, both immune-strengthening compounds increased. But when their blood was tested a week later after listening to the same piece of music — but without singing — its composition didn't change.

Sip a cup of tea: A study by Harvard researchers found that people who drank five cups of black tea every day for two weeks increased their production of virus-blasting interferon 10 times more than people who didn't drink tea. Researchers believe the primary active component is L-theanine, but tea is rich in a variety of disease-fighting polyphenols and flavonoids.

Laugh. Having a good belly laugh can significantly reduce levels of the stress-causing hormone cortisol, while at the same time increasing compounds that fight viruses.

According to a series of studies conducted at California's Loma Linda University, laughing increases T cells, natural killer cells, gamma interferon, immunoglobulin A, and gamma interferon — all compounds that activate the body's immune system, and increase the amount of antibodies that fight infection, cytokines that regulate your immune system, and killer cells that kill viruses. The researchers discovered that some of the immune changes were still present the following day.

Take vitamin D. Adequate levels of vitamin D are essential for a well-functioning immune system, but getting enough of the sunshine vitamin can be difficult in winter. A quick alternative is to pop a vitamin D tablet.

One randomized, double-blind study found that children who took 1,200 IU's of vitamin D every day lowered their risk of flu by 42 percent when compared to children who took a placebo. Some experts theorize that higher doses might yield even better results. One German study found that adequate vitamin D increases immune function as much as fivefold.

Wash Your Hands: Keeping your hands clean during the flu season is key to keeping viruses away from your body's entry points, such as your eyes, nose, and mouth. When you're out in public, use paper towels instead of a hand dryer and dry your hands thoroughly, since germs cling to damp hands more easily.