Even if used judiciously, even ample quantities will eventually be depleted. Without the ability to produce them synthetically, we will have to consider natural substances that might help alleviate various respiratory symptoms and strengthen the body's immune response.

Historically, Vitamin C, Vitamin E, and other antioxidants taken regularly are supposed to decrease the frequency and severity of respiratory infections.  Many studies confirm their usefulness, although the amount of down time due to colds/flus per year was only decreased 1 day in one study.  Despite this, antioxidant support of the immune system can be obtained through good nutrition or supplements and should be part of any survival food storage strategy.