Since bacteria have been known as the cause of some diseases, they have been commonly regarded as foes to human life. So they may prove to be if of disease-producing types or even if not, when they are consumed in large quantities. This is invariably the case when underprotected or overkept food is eaten, whether it be salads, milk, ice-cream, preserved food, water, or any easily contaminated substance that humanity consumes. But some bacteria there are which play a friendly role in nature by helping in production of foods that nourish humanity.

Dangers from bacteria can be averted when understood. Dust is bacteria-laden, hence the necessity of protecting from dust everything to be eaten, worn, or otherwise used by humanity. If the air breathed carries disease-germs it may cause disease. The air-supply needs therefore to be pure, free not only from excess of carbon dioxid but also from such bacteria as can harm humankind. Tuberculosis is spread by dried dust carrying the germs cast off by those diseased. Bacteria thrive in the soil many inches deep. The plant food-supply needs therefore to be soil-free as used. Water in passing through soil may take with it what soil contains. If sewage drains through to the water-supply, the water may contain the bacteria that may infest the intestinal tract. That of typhoid fever is one of these, so is borne by water to humanity. Only thoroughly boiled, filtered, or purified water is assuredly germ-free.

Some chemicals prevent bacterial growth, but they would usually also render a substance unfit for use as a food. Heat fortunately is also a destroyer of bacterial vitality. This makes cooking of importance in obtaining germ-free food. But all bacteria are not destroyed by the temperatures non-destructive to food, and some grow without air, so preserved foods can contain many bacteria, though freed of putrefactive bacteria.