Washing hands and social distancing are key to stopping the spread of infectious diseases like COVID-19, but a healthy diet plays a role, too. Here are nine foods that provide key nutrients to support your immune system, as well as three to avoid.

By Carolyn Williams, PhD, RD

The immune system is your body's great defender. It helps stop incoming attacks from viruses and bacteria. It helps your cells bounce back after illness. It can even help reduce the severity of an illness. That's why, especially when viruses and bugs are making the rounds in your community or even just in your home, you should pay a little extra attention to your immune system.

One of the best ways to care for your immune system and help make it stronger is with food — but not just any food. Certain nutrients in foods are vital for healthy immune functioning. While focusing on these foods is important when you're sick, it's just as important (maybe even more so) when you're healthy, because this enables your immune system to be at its strongest should it come into contact with harmful viruses or bacteria..

Check out these nine foods that provide those key immune-boosting nutrients, as well as three to avoid, to support your immune system.

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Foods That Can Help Boost the Immune System

1. Broccoli

A good source of vitamin C and beta-carotene, broccoli also contains sulfur compounds that research suggests may boost production of glutathione, an antioxidant compound. In terms of immune support, glutathione works by attacking free radicals to minimize their potential damage. This allows the immune system to focus on staying healthy, not repairing itself from damage. Other sulfur sources are most cruciferous vegetables that give off slight odor during cooking like cauliflower, bok choy, and kale.

2. Fortified Orange Juice

I'm not usually a proponent of drinking your fruit instead of eating it, but it's hard to beat the immune punch in fortified OJ. One cup naturally provides 100 percent of your Daily Value (DV) for vitamin C, an antioxidant vitamin that plays a primary role in keeping the immune system healthy, as well as 25 percent DV for vitamin D. This is important since most people have vitamin D levels below ideal, something that research suggests makes one more susceptible to illness. A 2017 study even suggests that supplemented vitamin D helps to prevent respiratory infections.

3. Eggs

Adequate protein intake is important to support immune response, and eggs are a great way to do this since they also contain nutrients like vitamin D, zinc, selenium, and vitamin E that the body needs for proper immune functioning. If you can, opt for eggs from chickens that were fed a vegetarian diet. You'll get ones with slightly higher levels of omega-3s and vitamins D and E. No need for organic or cage-free varieties though, as this doesn't appear to affect nutrient content.

Read More: All About Eggs: Grades, Safety, Nutrition & More

4. Bell Peppers

Citrus fruits are good sources of vitamin C, but if you really want to load up, go for a red or yellow bell pepper. A medium red bell pepper has more than twice as much vitamin C as a medium orange. Plus, bell peppers are packed with the antioxidant beta-carotene and have a small amount of vitamin E, also an antioxidant. Toss slices in a salad, stir-fry with other veggies, or use to dip hummus in place of pita bread.

5. Lean Beef

Surprised to find beef on this list? Here's why I consider it a top food to support immune health: a 4-ounce serving of flank steak provides more than half of the Recommended Dietary Allowance (RDA) for zinc, selenium, and vitamin B6. Getting adequate intake of these three nutrients can be challenging, and a slight deficiency of any may impair your immune system from working at 100 percent efficiency. This can increase your susceptibility to illness and lower your immune defense capacity. Look for ways to incorporate lean cuts of beef like sirloin, round steak, and flank steak up to three times per week.

Related: The Surprising Way to Make a Cheap Cut of Steak Taste Great

6. Spinach

Vitamin A is considered essential to normal immune functioning, and leafy greens like spinach are packed with beta-carotene, a form of vitamin A that is also an antioxidant. On top of that, leafy greens are a good source of folate, and some research suggests that a deficiency may impair immune response. Not a fan of spinach? Most dark green and orange vegetables are good sources of beta-carotene while folate is also in fruit, beans, nuts, whole and fortified grains.

7. Salmon

Immunity-boosting isn't a health perk often associated with salmon, but adequate and regular intake of omega-3s (particularly DHA and EPA, which are two forms found in cold-water fish) are key for tamping down inflammation. This, in turn, allows the immune system to focus more if its attention on defending the body against pathogens and fighting illness. Need a shelf-stable option? Stock the pantry with cans of salmon or light tuna in water. Both fish are also good sources of vitamin D and the antioxidant mineral selenium, two nutrients that also support immune health.

8. Yogurt

Consuming yogurt with minimal added sugar and "active, live cultures" is an easy way to support immune health. Yogurt's benefits come from the probiotics, or good bacteria, because research suggests the immune system and microbiome work with one another to target pathogens and to fine-tune immune responses. This means having an imbalance of good bacteria could potentially impact immune response effectiveness. Incorporate yogurt and other probiotic-rich foods to strengthen your microbiome, decrease gut permeability, and support immune function.

Related: Why You Need Prebiotics With Your Probiotics

9. Chicken Soup

It seems far-fetched that chicken soup could really do much for your health, but our grandmothers may have been on to something. Research suggests that eating chicken soup has a mild anti-inflammatory effect that impacts white blood cells. This could lead to a decreased risk of developing a respiratory infection, something that may be due to a compound in chicken that inhibits viral infections according to another study. While immune protection from chicken soup is definitely speculative, incorporating a bowl of chicken soup doesn't hurt and may offer more benefit over other hot liquids thanks to the protein, garlic, and onions. Another perk: making a large batch of soup saves time in the kitchen if working from home.

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Foods That Can Harm Your Immune System

Some foods can also impair your immune system from functioning at its best. Here are three to avoid or limit to minimize the negative impact they can potentially have on your body's health.

1. Caffeinated Beverages

A lack of adequate, good quality sleep impairs your body's immune response, increasing one's susceptibility to illness and decreasing immune defenses. And even if caffeine doesn't keep you from going to sleep, consuming more than three caffeinated drinks a day can prevent the body from spending adequate time in that deep, restorative sleep phase. To minimize these effects, limit caffeinated drinks to no more than two to three per day, and consume them during the first half of the day.

2. Processed Foods With Added Sugars

Many ready-to-cook packaged foods contain chemicals and compounds not naturally found in food like artificial colors, flavorings, preservatives, and/or added sugars to make them shelf-stable or to improve taste and appearance. Any of these can irritate the body to exacerbate existing inflammation. This in turn monopolizes immune response and decreases the body's capacity to defend against illness. Your best bet is to stick with whole and minimally-processed foods (five or fewer ingredients listed is often a good sign), and avoid unnecessary added sugars when possible.

3. Alcohol

Research suggests going beyond moderate consumption (more than one drink per day for women; more than two drinks per day for men) significantly impacts the body's immune response, increasing one's susceptibility to illness such as pneumonia and other respiratory conditions. Yes, "moderate "drinking offers small potential heart and anti-inflammatory benefits, but the key to drinking alcohol for these benefits while minimizing harm to immune defenses is to stay at or below "moderation" guidelines — or avoid all together.

Related: 6 Surprising Things Are Messing With Your Gut Health

Carolyn Williams, PhD, RD, is author to the new cookbook, Meals That Heal: 100+ Everyday Anti-Inflammatory Recipes in 30 Minutes or Less, and a culinary nutrition expert known for ability to simplify food and nutrition information. She received a 2017 James Beard Journalism award, and her work is regularly featured in or on respective websites for Cooking Light, Real Simple, Parents, Health, EatingWell, Allrecipes, My Fitness Pal, eMeals, Rally Health, and the American Heart Association. You can follow her on Instagram @realfoodreallife_rd or on carolynwilliamsrd.com.

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