The organisms of the kingdom Protista are usually classified as Protozoa, Algae, and Fungus-like Protists. Some useful organisms in this kingdom include a large number of species that are plankton; these form the lowest level in the aquatic food chain and help provide nourishment to organisms that lie in the higher levels. Another useful organism in this kingdom is a protozoa that forms a symbiotic relation with termites. It lives in their guts and helps in the digestion of cellulose found in the wood that is eaten by termites.
Harmful organisms of this kingdom include Entamoeba histolytica which is a parasite that lives in the colon and destroys the friendly bacteria. This leads to the ailment called amoebic dysentery. Other parasitic organisms include an example is Plasmodium vivax, which causes malaria, and destroys the red blood cells of the patient.
The useful organisms of the kingdom Fungi include all the mushrooms and truffles that are consumed as food. Fungi are also extensively used in the process of fermentation to make breads and alcoholic beverages.
Harmful fungi include the mold that destroy cooked food left in the open. Fungi also cause a lot of damage to cultivated plants and are the cause of many diseases in humans.