Eating healthy is the most important part of being healthy.
If you didn't accept that fact already, you should.
What you put into your body is everything; it's how you feel, how you look, and how likely you are to have most health issues. The coronavirus or any other infection is no different; the healthier your diet is, the less susceptible you will be to infection.
If you are eating a healthy diet full of different types of fruits, vegetables, vitamins, minerals, protein, and healthy fats, your body will contain a wider variety of the nutrients necessary to fight off infection. One example of this is with vitamin C and vitamin D3. Individuals who are deficient in these two vitamins have been shown to be more likely to contract infections. Someone who has been eating salmon, eggs, mushrooms, oatmeal, citrus fruits, broccoli, peppers, Brussels sprouts, and supplementing with a daily dose of vitamin D3 and vitamin C is going to be in a much better position to ward off infection than someone who consumes fast food and processed junk, devoid of these essential nutrients.
On the opposite end of the spectrum, the individual who is consuming sugary snacks and high fat, high carb foods is more likely to get sick and more likely to stay sick longer. Sugar inhibits immune system function, and bacteria actually feed off of it.
So how can you maximize your diet to have a strong immune system naturally? Consuming a variety of different nutrient-dense foods is important, to make sure you aren't missing out on any single vitamin or nutrient. If you do feel like you might not eat the largest variety (or want an extra boost), a multivitamin is a great daily supplement to act as a sort of "insurance policy," in case you find yourself falling back on the same meals every day.
Another great strategy is to consume as many antioxidant-rich foods as possible. This includes what you might think of as "superfoods" and typically they are rich in color--blueberries, strawberries, kale, beets, spinach, sweet potatoes, nuts, and fish are all great sources. Load your diet up with a few servings of these each day and you can't go wrong.
Once you focus on making your diet as nutritious as possible--not as low calorie, not necessarily maximized for fat loss, but maximized for health and well-being--you will likely be astounded by the results, not only related to immune system but also your energy levels, weight, and how you feel.
Make it your goal to consume a variety of superfoods and different nutrients you normally don't this week. Notice how you feel. Focus on bettering yourself and your health and you will come out of this stronger.