What Causes An Infection On A Cut Wound?

If my daughter gets any type of scratch or cut.. small or big it becomes badly infected with tiny yellow bubbles and makes her sick.. the cut gets blisters..take about 2-3 weeks to heal and leaves a big scar around the cut.. we have been back and forth to hospitals no one can explain it.. so many blood tests and no answers .. We are working with specialists and they are struggling to Understand what is wrong..