In 2020, COVID-19 shone a spotlight on zoonoses — diseases caused by pathogens that are naturally transmitted between humans and other animals. These pathogens can take the form of microorganisms, most commonly bacteria or viruses, or macroparasites, such as worms.

Since its beginnings 3.8 billion years ago, the biological world has been a web of interactions among organisms. Indeed, every living creature is, in fact, an amalgamation of other living creatures. Given this permanent web of interactions, certain microorganisms and parasites may end up in new host species, an erratic process facilitated by a range of factors. Sometimes these new relationships are beneficial. Sometimes they are catastrophic. Of the myriad interspecific exchanges taking place, very few ultimately succeed. The above should serve to remind us that all species host microorganisms. It is the nature of life. However, under certain circumstances, this otherwise banal reality can end up threatening the health of individuals and societies.

As human beings, we have complex relationships with animals. These connections differ across the world, as they are shaped by cultural practices, customs, traditions, and religious beliefs. Some animals are the objects of our affection. Others terrify us. In either case, animals are front and centre among our emotional connections to the living world. They improve our daily lives. Some provide us with joy, labour, or nourishment, while others simply share natural spaces with us. Each one of these interactions represents an opportunity for pathogen exchange. Certain pathogens are part of our evolutionary heritage because they were present in our great ape ancestors. The advent of domestication created an opportunity for frequent, routine contacts between humans and farm animals, thus favouring zoonosis transmission. Human and farm animal populations grew in tandem, reducing the relative representation of wildlife species among terrestrial vertebrates. At present, the ways in which we humans exploit the environment have increased how frequently we interact with wildlife. Simultaneously, intensive animal farming has transformed the conditions under which these interactions take place, with young and genetically homogenous animals crowded together at high densities.

In this book, we explore what is currently known about zoonoses, drawing upon multifarious examples. We seek to answer certain key questions: What are zoonoses? How are they transmitted? How do we learn to safely live with them? Are zoonoses on the rise? This book is an invitation to learn more about these diseases so that we can better protect ourselves and others. An essential part of this work is transforming how we interact with animals and the living world in general.