Primer on Disease

Bacteria: Bacteria are single cell organisms that occupy a large variety of ecological niches. The vast majority produce beneficial effects in our environment. Most bacteria have a cell wall that provides protection from their environment. Some bacteria have a capsule outside the cell wall — a polysaccharide (sugar) or polypeptide (amino acid) layer that is required for pathogenesis or disease. A host (plant or animal) may provide the environment and nutrients that allow bacteria to grow and in some cases cause disease. The ability of bacteria to cause disease is related to how well they invade a particular host and then defend themselves from the host’s defense mechanisms.

Viruses: As intracellular parasites, viruses can replicate only in living cells. In the environment, they are inert biochemical entities. Viruses are composed of nucleic acids, DNA or RNA (never the two in the same virus), surrounded by proteins and, for some viruses, by a fat-containing fragile envelope. Enveloped viruses remain infectious in the environment for much shorter periods of time than do nonenveloped viruses. This has a major influence on the spread of viral diseases. For example, the unstable, enveloped HIV virus requires extremely close sexual contact or exposure to virus-containing blood to move to the next host, as opposed to the non-enveloped foot-and-mouth virus that can move around the world on vehicles and remain infectious for months or years. Viruses attach to highly specific receptors on the cell membrane. The relationship between viruses and cell receptors defines whether a host is or is not susceptible to a particular virus. After attachment, viruses enter the cells, where they are reproduced by the cell’s metabolic machinery, and, eventually are released, ready to enter more susceptible cells. This process must happen for a virus to cause disease. The relation between hosts and viruses ranges from infection without consequence to mortality.

Mutation and other forms of germ evolution: DNA and RNA are the chemicals that make up genes. A set of genes that describes an organism is the genotype. When the instructions are copied and split to form a new organism, there can be an error in the copying. These changes are called mutations. Viruses can pick up cellular genes and deliver them to new cells, a process known as transduction. Bacteria can exchange DNA, a process called conjugation. All of these events can help an organism adapt to its environment, ward off antibodies/antibiotics and chemicals, or attach to (attack) different hosts, as when a disease of animals is suddenly able to attack humans. A change in DNA sequence can also weaken the organism or have no particular advantage or impact on the host.

Identifying germs: There are various tests to identify disease agents. In rare instances, symptoms give the disease away. Most of the time, however, clinical symptoms or lesions only suggest the presence of an agent, which must be specifically identified. Viruses and bacteria are identified by hundreds of various laboratory procedures. However, for the level of detective work and response being discussed in this article, scientists are identifying organisms by their genotypes and at an even finer level of detail. Determining the nucleic acid (nucleotide) sequence of the DNA allows identification of mutations and discovery of which proteins or sequences render the agent pathogenic and which sequences allow an organism to attach to a particular host.

Antigen:An antigen is any substance foreign to the host, such as a virus, bacterium, or components of a virus or bacterium, that elicits an immune response. An immune response may be humoral, i.e. antibodies circulating in the blood and tissue fluids, or cellular, represented by specific cells capable of attacking the infectious agents themselves or of attacking cells that have been infected by these agents. The latter capacity is particularly important for viral diseases, as these agents can replicate only in living cells.

Vaccine: A vaccine is a preparation of modified (attenuated) or killed organisms or components of these organisms that, when injected, stimulate an immune reaction that results in protection of the host. Thus the body is primed with antibodies or with stimulated specific cells to respond should the disease-causing organism invade. In live-attenuated vaccines, the microorganism is too weak to cause disease but is capable of stimulating the immune system.