Bacteria are prokaryotes. Their cells do not include a nucleus or membrane-bound organelles. Many species of bacteria can be put into one of two groups, Gram positives and Gram negatives, based on their cell wall structure. Gram positive bacteria have many layers of peptidoglycan in their cell walls and include common bacteria like Staphylococcus aureas, Streptococcus pneumoniae, Bacillus anthracis, Lactobacillus acidophilus found in yogurt, and Chlostridium botulinum. Gram negative bacteria have fewer layers in their cell wall and have an extra membrane surrounding their cell wall. Gram negatives include E. coli, Psuedomonas aeruginosa, Proteus mirabilis, Enterobacter aerogenes, and Neisseria gonorrhoeae.
Fungus includes both yeast and mold. One common yeast is Saccharomyces cerevisiae. It is used to make bread and alcoholic beverages. Molds include Rhizopus spp., Aspergillus spp., and Penicillium spp.
Viruses are obligate intracellular parasites, which means that they cannot reproduce outside of a host cell. Viruses include varicella-zoster virus (chicken pox and shingles), HIV (AIDS), herpes simplex virus (cold sores and genital herpes), human papilloma virus (warts), and bacteriophages, which infect bacteria.
Parasites are organisms that live in or on a host and benefit from it while causing harm to the host (optimally) without killing it. Entamoeba, Giardia, Plasmodium, flukes, tapeworms, ticks, fleas, and lice are all considered to be parasites.