Zinc is good for boosting the immune system. A lack of Zinc will mean that you get sick more often and take longer to get better. For the experts opinion check out this link:
I should add...at 15 her body is going through a lot of changes and is pretty vulnerable to illness. Zinc will help stave this off and should help keep other things like spots at bay.
Its well known that Vitamin C is good at boosting the immune system, but a lot of that good is undone by not having enough Zinc. Zinc is used in the body to help process the Vitamin C and make sure its used correctly.
Apparently a valid rule of thumb to see if you need to boost your zinc intake is to dissolve an unflavoured zinc tablet in water. if your body has enough zinc. you will taste it in the water, if it hasn't, you won't. Strange but (I understand) research based!!