Our ancestors evolved over millions of years to survive in their surroundings. For most of man's existence, this environment has been characterized by extreme challenges, such as food shortages, or food that can carry disease, as well as unhygienic conditions and dirty water, extinction of time, and so on. It was a dangerous environment, something to survive.
At the center of our defense was our immune system, our most elegant defense. The system is a product of the centuries of evolution, since the river stone is shaped by water that moves above it and falls on the flows of its journey along the stream.
Late in the process, people learned to take steps to strengthen our defense, developing all kinds of customs and habits to support our survival. In this way, I mean the brain – the organ that helps us develop habits and customs – as another aspect of the immune system.
We used our collective brains to learn effective behavior. We started to wash our hands and take care to avoid certain types of food, for which the experience shown can be dangerous or deadly. In some cultures, people came to avoid pork, which we now know is highly susceptible to trichinosis; in others, people banned meat, which we later learned can carry toxic loads of E. coli and other bacteria.
Ritual washing is mentioned in the Exodus, one of the earliest books in the Bible: "And they will wash their hands and feet, that they may not die"
Our ideas have evolved, but for the most part, the immune system does not. This does not mean that it has not changed. The immune system reacts to our environment. When faced with various threats, our defenders learn, and then they are much more able to cope with this threat in the future. That way, we adapt to our environment.
We survived tens of thousands of years. Finally, we washed our hands, we covered the floors, prepared our food, we avoided individual foods. We improved the hygiene of the animals we collected and sacrificed for food.