Thanks for your query
Your doctor is right in saying that your child is catching infection.
As you said none of your other child has this but every child is different.
So i suggest you to maintain proper hygiene,as she is getting recurring infection you can consult your doctor for blood test
In the meantime apply calamine
over lesions 3 to 4 times a day.she needs a mild steroid
for local application,
Wishing your child good health.