kitsune,
Oh no, not fruit juice, esp not the commercial/bottled one. Less sugary stuff, remember what tongtong said? Commercial fruit juice has been added with too much sugar. Unless you juice it yourself (but then wasted those fiber).
One orange a day is okay lah. I give my kids oranges even when they were coughing, no bad effect for them. To keep fruits, eg, strawberries for longer, wash/rinse them thoroughly then dry it up and keep in air tight container. They are other fruits like papaya, banana, guava, pear. I don't believe supplement can boast immunity. Cut down processed food as much as possible, drink plenty of water, enough sleep, and exercise.