Are her glands swollen a lot? Has her Dr checked her tonsils? I can say from experience my daughter around that same age was sick alot. You don't to invest in Juice Plus or anything expensive. I mean kids get sick. Kids that are exposed to things the first time are hit the hardest. Kids in preschool get hit hard and as their immune systems build up they are a lot better for it once they hit public school. I have two kids so I am very aware.
My daughter ATE VERY HEALTHY (still does) HOWEVER she had mild sleep apnea and her glands seemed to be very large. They took her tonsils out and her adnoids. She has rarely if ever been sick since and is going on 8. She missed only two days of school this past year and one she wasn't even really that sick. Making the assumption she is not getting her nutrition is presumptious I am sorry by the other posters. Some kids are more susceptible. I now give my kids Airbornne for kids the first sign of a cold, which either gets rid of it or lessens the symptoms. My son was sick a lot around 2 as his sister brought things home, once we got over the hump he too hasn't been sick hardly at all. It takes a while and in public there are precautions, wiping the cart with a anteseptic wipe, hand sanitizer, hand washing but you cannot prevent it all over the place either.
It is important to have them eat well, however it is bull that is the cause. I have friends who's kids are so picky they eat very little and are very healthy. Kids overall eat way healthier then we did as kids. You can provide supplements if your Dr okays it and she doesn't eat well.
Chart her illnesses, see if you see a pattern, then discuss it with her Dr. If her tonsils are holding infection, her immune system is on overload and will not completely heal. She is getting something on top of something else because she hasn't had time to recover the first time.
My daughter never had strep throat, when her tonsils were removed the Dr was blown away how large they were and held infection. Now they are out, she is like a new kid. Sleeps better and fully at night and hasn't been sick but a handful of times over the past five years.