Microbe types:

Viruses - Acellular, ultramicroscopic biological entities that act as obligate intracellular parasites.  All viruses are pathogenic.

Fungi - Eukaryotic (have a nucleus and membrane-bound organelles), uni- and multicellular heterotrophic organisms having a cell wall composed of chitin. Fungi reproduce primarily through sexual or asexual spore formation.  Fungi exist in two basic forms, yeasts which are single-celled and molds, which are composed of long filaments of cells called hyphae.  Masses of hyphae form the main body structure of a mold, called the mycelium.  Most fungi are nonpathogenic saprophytes (consume dead tissues), but some are pathogenic.

Protists - These are eukaryotic microbes that share characteristics with plant, animal or fungal cells.  Uni- and multicellular forms exist.

a.    Protozoa are unicellular animal-like protists that move through the use of
       structures such as pseudopods, flagella and cilia.  Most are free-living,
       though many form symbiotic relationships with other organisms. Some,
       such as Trypanosoma gambiense, Plasmodium falciparum, Balantidium
       coli, Entamoeba hystolytica, Pfisteria and Giardia lamblia are
       pathogens.

b.    Algae are plant-like protists.  Many are unicellular but some, such as
       Laminaria (kelp) are multicellular (kelp can grow up to 100 feet in
       length!) Euglena is a unicellular plant-like protist that has a chloroplast
       but also has a gullet it can use to ingest large food particles.  Though
       none of the algaes are pathogens, one group called the dinoflagellates
       (fire algae) includes members such as Gonylaux, a unicellular organism
       responsible for red tide.  Red tide occurs when dinoflagellates grow out
       of control (bloom) and release toxins that kill fish and cause paralytic
       shellfish poisoning (ciguatera) in humans.

c.    Fungus-like protists include both cellular and acellular slime molds.
      Most of these are free-living saprobes, but some, such as
      Phytopthera infestans cause diseases of plants such as late blight
      of potato and Saprolegnia, which is responsible for the disease ick
      in fish.

d.  Bacteria and Archaea are prokaryotes (lack a nucleus or membrane-
     bound organelles).  Most of these organisms are much smaller than
     eukaryotic cells and all are unicellular, though many can form
     associations with other cells such as clusters or chains.  Bacterial
     cell walls are composed of a complex sugar called peptidoglycan,
     while archeal cell walls are composed of other organic polymers.
     The Archaea are often found in very extreme environments such as
     alkaline or acidic water, along volcanic vents and even in rock.

e.  Helminth worms belong to Kingdom Animalia, but are parasites
     often studied by microbiologists.  Tapeworms, the sheep liver
     fluke, Ascharis, Trichinella, Enterobius and Trichina still are
     causes of disease among human and animal populations today.