Humans and animals have always lived with bacteria, viruses, fungi, and parasites.

Some of these are very useful, “actively collaborating in numerous vital functions.” Others are pathogenic (causing disease) or can become so on certain occasions.

Studying the structure and functions of microorganisms (genetics, metabolism, interactions), their ability to evolve or adapt (environmental adaptation, drug resistance, ability to penetrate animal cells and interact with or destroy them) forms the basis for preserving or restoring health and for understanding many diseases.