Viruses are obligate intracellular parasites; in other words, in order to replicate their genetic material and thus multiply, they must gain access to the inside of a cell. Viruses are often considered non-living for several reasons:

The genetic material for viruses may be either DNA or RNA, never both. The nucleic acid core is encapsulated by a protein coat (capsid) which together forms the head region in some viruses. The tail region helps to anchor the virus to a cell. An extracellular viral particle is called a virion. Viruses are much smaller than prokaryotic cells (ie. bacteria) which, in turn, are much smaller than eukaryotes (ie. animal cells, fungi). A virus which infects bacteria is called a bacteriophage or simply a phage.

The life cycle of viruses has many variants; the following represents the main themes. A virus attaches to a specific receptor on a cell. Some viruses may now enter the cell; others, as in the diagram, will simply inject their nucleic acid. Either way, viral molecules induce the metabolic machinery of the host cell to produce more viruses. The new viral particles may now exit the cell by lysing (bursting) it. The preceding is deemed lytic or virulent. Some viruses lie latent for long periods of time without lysing their host cell. These are called lysogenic or temperate viruses.

Prokaryotes are organisms without a membrane bound nucleus. They are haploid and have a long circular strand of DNA in a region called the nucleoid. Bacteria also have smaller circular DNA called plasmids which help to confer resistance to antibiotics. Bacteria do not have mitochondria, Golgi apparatus, lysosomes, nor endoplasmic reticulum. Instead, metabolic processes can be carried out in the cytoplasm or associated with bacterial membranes. Bacteria have ribosomes (smaller than eukaryotes), plasma membrane, and a cell wall. The cell wall, made of peptidoglycans, helps to prevent the hypertonic bacterium from bursting. Some bacteria have a slimy polysaccharide mucoid-like capsule on the outer surface for protection.

Bacteria are partially classified according to their shapes: cocci which are spherical or sometimes elliptical; bacilli which are rod shaped or cylindrical; spirilli which are helical or spiral. They are also classified according to whether or not their cell wall reacts to a special dye called a Gram stain; thus they are gram positive if they retain the stain and Gram negative if they do not.

Most bacteria engage in a form of asexual reproduction called binary fission. Two identical DNA molecules migrate to opposite ends of the cell as a transverse wall forms, dividing the cell in two. The cells can now separate and enlarge to original size. Under ideal conditions, a bacterium can undergo binary fission every 10-20 minutes producing over 10³⁰ progeny in a day and a half. If sources are unlimited, exponential growth would be expected.

Bacteria do not produce gametes nor zygotes, nor do they undergo meiosis; however, three forms of genetic recombination do occur: transduction, transformation, and conjugation. In transduction, phages act as a vector for transferring DNA between bacteria. In transformation, bacteria incorporate free DNA from its immediate environment (ie. from a dead cell which has released its DNA). In conjugation, part of the DNA strand may be passed from one mating type to another through a hollow tube (called the pilus) while the two cells are incontact.

Most bacteria cannot synthesize their own food and thus depend on other organisms for it; such a bacterium is heterotrophic. Most heterotrophic bacteria obtain their food from dead organic matter; this is called being saphrophytic. Some bacteria are autotrophic meaning they can synthesize organic compounds from simple inorganic substances. Thus some are chemoautotrophic obtaining energy via chemical reactions including the oxidation of iron, sulfur, nitrogen, or hydrogen gas.

Bacteria can be either aerobic or anaerobic. The former refers to metabolism in the presence of oxygen and the latter in the absence of oxygen (ie. fermentation). An obligate anaerobe would die in the presence of oxygen, whiel a facultative anaerobe would survive.

Fungi are eukaryotic organisms which absorb their food through their chitinous cell walls. They may either be unicellular (ie. yeast) or filamentous (ie. muchrooms or molds) with individual filaments called hyphae which collectively form a mycelium.

Fungi often reproduce asexually. Spores (ie. conidia) can be produced and then liberated from outside of a sporangium; or, as in yeast, a simple asexual budding process may be used. Sexual reproduction can involve the fusion of opposite mating types to produce the asci (singular: ascus), basidia (singular: basidium), or zygotes. All of the three preceding diploid structures must undergo meiosis to produce haploid spores. If resources are unlimited exponential growth would be expected.

Fungi are relatively important for humans as a source of disease and a decomposer of both food and dead organic matter. On the lighter side, they also serve as food (mushrooms, truffles), for alcohol and food production (cheese molds, bread yeast) and they have given us the breakthrough antibiotic penicillin (from penicillin molds).