The rapid increase in the prevalence of cancer and the critical importance of the environment as a factor in cancer prevention suggests a simplistic view, in fact most sporadic cancers are environmental in their etiology.

It is still unclear how the environment triggers the onset of cancer in many types of tumors, new technologies have allowed studying the complex ecosystems of bacteria, fungi, viruses and parasites that inhabit the human body. For many years these organisms were seen as passengers causing infections, not as agents of creating a chronic disease. However, it is now clear that the relationship between the host and its vast and very diverse population of organisms is profound, complex and essential to almost all aspects of human health. This relationship is based on hundreds of thousands of years of coevolution that result in physiological requirements of the host that can only be met by the microbiota. In other words, humans are superorganisms of genomes of massive interconnection of trillions of organisms that are essential for maintaining health. Therefore, cancer has been studied with significant details from this “systems” perspective, since it represents a clear example of how microbes can cause chronic diseases and it is well established that microbes are the key factors of certain types of cancer; like  gastric or colorectal.