I make my own EIS silver the same way. I gargle it in the throat and drink up to 8 ounces per day when sick. Silver isn't toxic so you just make sure you get enough.
The reason they probably get sick in the winter is because of lack of sunshine which the skin uses to make vitamin D3 . 6000 mg per day of D3 will take care of the deficiency.
Lugols Iodine and cayenne pepper are other immune system boosters. Learn to use them.