Ah, January, the time when you pack away the holiday lights, put away the presents and then dive, stuffy head-first, into flu season.Flu activity usually peaks in the U.S. in January or February. According to the Centers for Disease Control and Prevention, influenza activity is on the rise for the season in the U.S., with 29 states experiencing high levels of influenza-like illness.What can you do to prevent the flu? Practice good hygiene like washing your hands frequently, including before you touch your face or eat.If you have not been vaccinated against the flu, you should get that done now. If you have severe influenza illness, or are at risk for influenza illness-related complications, you should get the influenza antiviral medications (regardless of whether you've had the flu vaccine).In addition, there are a number of holistic interventions you can do to boost your immunity before or during a flu or upper-respiratory infection.Here are our top holistic immunity-boosting tips for the flu season:Decrease your intake of processed carbohydrates and sugary foods. Even small amounts of sugar can suppress the immune system and make you more likely to get an infection, or more sick when you have one.The herbs Echinacea angustifolia and Echinacea purpurea are for prevention and long-term immune support. Doses generally used are 2.5 grams of root taken preventively, and this can be tripled to head off an impending infection.The herb Andrographis paniculata is used in acute viral or bacterial infections. Doses used are typically 1,000 milligrams per day of extract, or about 6 grams of the herb.The herb Ashwagandha is considered an adaptogen, and can help the body respond to stress and ward off illness. It can also help insomnia related to the flu if taken in the evening. Doses of Ashwagandha are about 1,000 milligrams a day, or a couple pills per day when the pill doses are about 400 to 500 milligrams.Fresh ginger steeped in hot water to form a hot tea can be a natural immune booster, and can help fight nausea, congestion and abdominal bloating. You can add a little fresh lemon and honey to the tea to calm a sore throat.Nasal saline rinses, or neti pots, used daily can help wash out viruses and bacteria in the nasal passages.Garlic -- raw, cooked or in the form of supplements -- can boost immunity and has been shown to reduce cough and congestion.Doses of garlic recommended are 2 to 5 grams of fresh raw garlic, 0.4 to 1.2 grams of garlic powder, or 2 to 5 milligrams of garlic oil daily.Garlic can be taken with food to minimize gastrointestinal upset.Exercising in moderation has been shown in many studies to be a natural immunity booster, and may reduce the incidence and severity of viral infections.If you think you have the flu, contact your physician.Drs. Kay Judge and Maxine Barish-Wreden are medical directors of the Sutter Downtown Integrative Medicine program in Sacramento, Calif.