Bacteria are a large group of unicellular microorganisms found growing in soil and other organic matter and the live bodies of plants and animals. Despite their apparent simplicity, bacteria can form complex associations with other organisms, categorized into parasitism, mutualism and commensalism. Bacterial diseases are also important in agriculture, with bacteria causing leaf spot, fire blight and wilts in plants, as well as Johne's disease, mastitis, salmonella and anthrax in farm animals.
Most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil, on dead matter, and as symbionts of plants, animals, or other fungi but may become noticeable when fruiting, either as mushrooms or molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. Fungi are also used as biological agents to control weeds and pests.
A virus is an infectious agent too small to be seen directly with a light microscope. They are not made of cells and can only replicate inside the cells of another organism (the viruses' host). Viruses infect all types of organisms and are found in almost every ecosystem on Earth. Viruses spread in many ways; plant viruses are often transmitted from plant to plant by insects that feed on sap, while animal viruses can be carried by blood-sucking insects or simple contact with the exchange of fluids.