An interesting in-depth book about the air we breathe and all the microbes in it was recently published. There are multitudes of microbes (viruses, bacteria, fungi) hidden in our air. We can't see them, but they are there. The book Air-borne: The Hidden History of the Life We Breathe is by award winning author Carl Zimmer.
Zimmer discusses a wide range of topics in this book, including how the virus COVID moves through the air (and its origins), the development of biological weapons (e.g., anthrax, smallpox) meant to spread through the air, the history of what humans know about microbes in the air, how air moves globally for thousands of miles, and the movement of a variety of contagious viruses through air. There is also a reference section at the end of the book.
Carl Zimmer - Air-Borne: The Hidden History of the Life We Breathe (published by Dutton, 2025).
By the way, every day we all breathe in about two thousand gallons of air, which contains bacteria, fungi, and viruses. This is part of life.