The science of microbiology is study of microorganisms and their activities. The word microorganism originates from the Greek word micro, meaning small. Microorganisms are very small life forms—so small that individual microorganisms usually can only be seen with magnification. The major groups of microorganisms are bacteria, algae, fungi, viruses, and protozoa. All are widely distributed in nature. Most microorganisms are smaller than 0.1 mm, so we need a microscope to increase the resolving power to make the microorganisms visible. Microorganisms have provided specific systems for the investigation of the physiologic, genetic, and biochemical reactions that are basis of life. Microbiologists have been remarkably successful in explaining the useful microorganisms and in combating the harmful ones. Considering the small size of the microorganism, it is no wonder that the existence of microorganisms was not recognized until a few centuries ago.