building up immune system in my dog?

Use vitamin c, a dash of the powder form on food.

A bit of raw garlic, about a small pinch.

A good quality food, I recommend Orijins. Nothing with corn, wheat, or any grain for that matter. And avoid the big names like ALPO, they used rendered dead dogs and cats in the food. Now you know what "animal byproducts" are. (more info if you search this on google)

Keep your pooch stress free and relaxed, as stress is rough on the immune system. Keep them warm and dry too.

You can also talk to your vet regarding things like a multi vitamin. And you can feed fish oils and olive oil (in dashes on the food) to help your pet be top notch.

Keep your dogs vaccinations current and feed good food. I would not accept a foster dog who is obviously contagious - sometimes vaccinations are not enough, and you have an obligation to your dogs to protect them. I give every new foster dog a bath the first night, and worm him. I also crate the foster dog at night for the first week, and do not allow him up on the furniture, so if there are any skin problems, they will not be readily shared. Once he is there for a while, and its clear the dog is housebroken, then he is allowed to share bedding and furniture.

There are several supplements on the market but I use Echinacea/Goldenseal for my dogs and myself. Top food and exercise helps too.