1) Microorganisms are organisms too small to be seen with the naked eye, including bacteria, archaea, protozoa, algae, fungi, viruses, and some multicellular organisms.
2) Microorganisms can be divided into dangerous types that cause disease and useful types employed in industries like food production.
3) Dangerous microorganisms include E. coli, C. botulinum, salmonella, cholera, tetanus, staphylococcus, syphilis, dengue fever virus, yellow fever, and HIV, all of which can cause severe illness or death in humans.