Microorganisms (or microbes) are living organisms that are generally too small to be seen without a microscope. Throughout history, humans have used microbes to make fermented foods such as beer, brea...Microorganisms (or microbes) are living organisms that are generally too small to be seen without a microscope. Throughout history, humans have used microbes to make fermented foods such as beer, bread, cheese, and wine. Long before the invention of the microscope, some people theorized that infection and disease were spread by living things that were too small to be seen. They also correctly intuited certain principles regarding the spread of disease and immunity.