A virus is an acellular infectious agent consisting of a nucleic acid strand within a protein coat. Because viruses are acellular, or without a cell, they are not considered alive. Viruses are considered microbes because of their small size. Viruses cannot reproduce without a host—an organism on which they rely for metabolic processes. Thus, viruses do not have a taxonomic classification. Instead, viruses are classified according to their own system, which is still under development and tends to group viruses according to the composition of their nucleic acids, or the morphology and composition of their protein coats. The genomes of viruses are made of nucleic acids, but they may be DNA or RNA, and single stranded or double stranded. These genomic distinctions are one method of classifying them. Similarly, virus shape is variable and can be used for classification purposes. For example the tobacco mosaic virus is helical while the rhinovirus causing the common cold is icosahedral. Viroids, like viruses, are acellular infectious agents, but they are composed exclusively of a single stranded RNA molecules and infect only plants.
A fungus (plural, fungi) is a eukaryotic organism, possessing a nucleus and membrane-bound organelles, that may be unicellular or multicellular and that produces spores and contains chitin in its cell walls. Mushrooms are a form of multicellular fungus that can be seen without magnification. Depending on the fungi, reproduction can occur asexually, sexually, or for example in the corn smut fungus, Ustilago maydis, alternate between asexual and sexual reproduction. Fungi are heterotrophic, meaning they obtain their food from other organisms. Some types of microbial fungi can cause infection, such as the various fungi that cause the condition known as athlete's foot, a fungal infection of the skin of the feet.
A protozoan (plural, protozoa) is a unicellular, eukaryotic organism classified into the kingdom Protozoa. Protozoa are in the domain Eukarya, meaning they have nuclei and membrane-bound organelles. Protozoa are exclusively heterotrophic, so they must obtain nutrients from other organisms, with the exception of Euglena, which is capable of photosynthesis. Protozoan reproduction can be asexual or sexual. Many protozoa consume bacteria to prevent overgrowth. Some protozoa are pathogenic to humans; for example, Giardia is a pathogen that causes the intestinal illness giardiasis, and Plasmodium is the genus of pathogens that cause malaria.
A helminth is a large, multicellular wormlike organism that is visible with the naked eye when fully mature and belonging to the phylum Nematoda or Platyhelminthes. The microscopic eggs and larvae of helminths are infectious agents, which is why these multicellular worms are included in microbiology. Helminths are not a taxonomic classification but refer to parasitic worms from several different phyla, including Platyhelminthes, Nematoda, Annelida and Ancanthocephala . Most helminths are pathogenic; for example, tapeworms and roundworms are often found in intestinal tracts of mammals. The larva of the pork tapeworm is ingested from improperly prepared pork; it attaches to the small intestine and grows into an almost 10-foot-long worm.