Microorganisms are a large class of organisms, including bacteria, viruses, fungi, some small protozoa, microalgae and so on. Although they are small, they are closely related to human life. Covers a variety of beneficial and harmful, covering health, food, medicine, industry and agriculture, environmental protection and other fields.

One of the most important effects of microorganisms on human beings is the prevalence of infectious diseases. 50% of human diseases are caused by viruses. The history of human diseases caused by microorganisms is also the history of human struggle against it. Great progress has been made in the prevention and treatment of diseases, but new and reappearing microbial infections continue to occur, for example, a large number of viral diseases have been lacking effective therapeutic drugs.

There are many kinds of microorganisms, some of which are corrupt, that is, they cause bad changes in food odor and tissue structure. Of course, some microorganisms are beneficial. They can be used to produce cheese, bread, pickles, beer and wine.

Microorganisms are so small that they must be magnified with a microscope to see them. For example, for medium-sized bacteria, 1000 is only as big as a period. Imagine a drop of milk, there are about 50 million bacteria in every milliliter of rotten milk, or the total number of bacteria in every quart of milk is about 5 billion. That is, a drop of milk may contain 5 billion bacteria.

Microorganisms can cause diseases, which will cause food, cloth, leather and other moldy rot, but microorganisms also have a beneficial side. It was Fleming who first discovered penicillin from Penicillium which inhibited the growth of other bacteria, which was an epoch-making discovery in the medical field. Later, a large number of antibiotics were screened from the metabolites of actinomycetes. The use of antibiotics saved countless lives in World War II.

Some microorganisms are widely used in industrial fermentation to produce ethanol, food and various enzyme preparations. Some microorganisms can degrade plastics and treat wastewater and waste gas. , and has great potential of renewable resources, known as environmental microorganisms; Some microorganisms can survive in extreme environments such as high temperature, low temperature, high salt, high alkali and high radiation, and some microorganisms still exist.

It seems that many microorganisms have been discovered, but in fact, due to the limitation of technical means such as culture methods, the microorganisms discovered by human beings today only account for a small part of the existing microorganisms in nature.