You can do some things to help prevent the cold virus or flu virus from infecting you at least somewhat but you cannot usually prevent the cold virus or flu virus all the time.
To help protect your body from getting a cold virus or flu virus make sure to get plenty of Vitamin C, wash your hands often using antibacterial soap, take cold medicine before the onset of a cold.
Eat plenty of green vegetables and drink green tea as well as the vegetables contain vitamins and the tea helps to detox your body due to the antioxidants.
Be sure to get plenty of rest as well to help improve your health because when you don't get enough sleep your bodies immune system also becomes weakened.
Also get plenty of exercise because exercise is very healthy for you in other ways as well and your body will be able to fight off infections better when you're fit.
Eating some hot and spicy foods and Jalapenos can help boost your immune system as well but just don't overdo the hot and spicy foods or Jalapeno peppers because then you could have stomach issues.