Vitamin D and wash your hands often. Not just a quick rinse, sing the Happy Birthday song in your head while you wash with soap for a good wash. And clean the shopping cart handles with the disinfectant towelettes they provide.
I used to do the clean the carts with disinfectant thing....then, decided, or heard somewhere, that those germs are actually building up your natural immunity.
I just spray my hands, with alcohol spray, when I get in the car and wash them, good, once home.