Introduction

Alongside the plants and animals we can see, microbes are a huge, often invisible part of every biological system on Earth. You met their diversity in Class XI — bacteria, fungi, protozoa, and microscopic plant and animal viruses, plus viroids and prions, are found virtually everywhere: in soil, water, air, inside other organisms, and even in places no other life survives, such as boiling geysers, deep soil layers, thick snow, and highly acidic environments. Bacteria and many fungi can even be grown on nutrient media to form visible colonies, which is how they're studied in the lab.

The previous chapter covered how microbes cause disease — but that's only part of the picture. Most microbes are not harmful at all; a great many are directly useful to us. This chapter looks at some of their most important contributions to human welfare.