My oldest keeps getting sick...

My 6 year old daughter keeps getting sick from school, it is literally something new/different every week or two. I don't understand, are we doing something wrong? I have her wash her hands with soap, use hand sanitizer etc...always! She started school (kindergarten) this year in August and she literally has not caught a break. She was at home with me for the first 5 years of life as I'm a SAHM, I get her immune system needs to get stronger etc..but is this normal for her to constantly be sick it's been 4 months!? I feel bad for her and I know it's not her fault, but we are getting a little irritated from how often it is, as I also have a 15 month old and she'll most of the time catch what she has as well and I'm due with baby #3 any day now and I don't want the new baby to catch all this.😔 Is there anything we can do or you recommend!? Since her doctor says this is normal for her age, and it could take a year!😢😭