What am i doing wrong with my kids?

i have 3 kids btwn the ages 5 months and 5 yrs. they are almost alwayz sick! my son who is 5 has seasonal asthma and allergies and had tubes put in his ears 1 yr ago and just got an ear infection. i feel like i have done this to him somehow. like i'm not protecting him enough from getting sick. i don't know what more to do. my 3 yr old has has strepp throat like 3 times in the last 3 months and now has strepp throat and an ear infection and thats not supposed to happen. also has allergies. my new born has acid reflux and is congested alot. i was told she would have asthma too but worse than my 5 yr old. my husband gets fustrated cuz they are alwayz sick and although he has not said it i feel like he thinks its my falt cuz i am alwayz with the kids and take care of them most. please can anyone give a confused mother some advice on how to keep them healthier. i give them vitamins and they eat fruit and sometimes veggies but i subtitute veggies with the veggie juice.