Even in ancient times, scholars believed that diseases could be spread by organisms too small to be seen by the naked eye. Before we discovered that bacteria cells were the real culprits, many attributed disease to other sources. Now that scientists have definitively identified the microscopic causes of various infectious diseases, microbiology, or the study of microscopic-sized organisms, has become an increasingly important field in biology and in the larger biomedical community. Most microbes are harmless. Some of them are essential for life on Earth, e.g. through their ability to fix nitrogen. Biotechnology, which is truly the industry of our times, takes advantage of microbes for the production of a variety of complex substances, and it also mass-produces natural and engineered microbes for human use.