Viruses are subcellular organisms made up only of a ribonucleic acid (RNA) nucleus or a deoxyribonucleic acid (DNA) nucleus covered with proteins. They’re the smallest known organisms, so tiny that only an electron microscope can make them visible. Independent of the host cells, viruses can’t replicate. Rather, they invade a host cell and stimulate it to participate in forming additional virus particles. Some viruses destroy surrounding tissue and release toxins. Viruses lack the genes necessary for energy production. They depend on the ribosomes and nutrients of infected host cells for protein production. The estimated 400 viruses that infect humans are classified according to their size, shape, and means of transmission, such as respiratory, fecal, oral, and sexual.

Retroviruses are a unique type of virus that carries their genetic code in RNA rather than the more common carrier DNA. These RNA viruses contain the enzyme reverse transcriptase, which transcribes viral RNA into DNA. The host cell then incorporates the alien DNA into its own genetic material. The most notorious retrovirus known today is human immunodeficiency virus.

Bacteria are simple one-celled microorganisms with a cell wall that protects them from many of the defense mechanisms of the human body. Although they lack a nucleus, bacteria possess all the other mechanisms they need to survive and rapidly reproduce.

Bacteria can be classified according to shape — spherical cocci, rod-shaped bacilli, and spiral-shaped spirilla. They can also be classified according to their need for oxygen (aerobic or anaerobic), their mobility (motile or nonmotile), and their tendency to form protective capsules (encapsulated or nonencapsulated) or spores (sporulating or nonsporulating).

Bacteria damage body tissues by interfering with essential cell function or by releasing exotoxins or endotoxins, which cause cell damage.

Fungi have rigid walls and nuclei that are enveloped by nuclear membranes. They occur as yeast (single-cell, oval-shaped organisms) or molds (organisms with hyphae, or branching filaments). Depending on the environment, some fungi may occur in both forms. Found almost everywhere on earth, fungi live on organic matter, in water and soil, on animals and plants, and on a wide variety of unlikely materials. They can live both inside and outside their host.

Mycoplasmas are bacterialike organisms, the smallest of the cellular microbes that can live outside a host cell, although some may be parasitic. Lacking cell walls, they can assume many different shapes ranging from coccoid to filamentous. The lack of a cell wall makes them resistant to penicillin and other antibiotics that work by inhibiting cell wall synthesis.

Rickettsia are small, gram-negative, aerobic bacterialike organisms that can cause life-threatening illness. They may be coccoid, rod-shaped, or irregularly shaped. Rickettsia require a host cell for replication. They have no cell wall, and their cell membranes are leaky; thus, they must live inside another, better protected cell. Rickettsia are transmitted by the bites of arthropod carriers, such as lice, fleas, and ticks, and through exposure to their waste products. Rickettsial infections that occur in the United States include Rocky Mountain spotted fever, typhus, and Q fever.

Spirochetes are an atypical type of bacteria that have a helical shape, and the length is many times its width. They have filaments wrapped around the cell wall that propel the spirochete in a corkscrew motion. Spirochetes occur with Lyme disease and syphilis.

Parasites are unicellular or multicellular organisms that live on or within another organism and obtain nourishment from the host. They take only the nutrients they need and usually
don’t kill their hosts. Examples of parasites that can produce an infection if they cause cellular damage to the host include helminths (such as pinworms, roundworms, and tapeworms) and arthropods (such as mites, fleas, and ticks). Helminths can infect the human gut; arthropods commonly cause skin and systemic disease.

The body has many defense mechanisms for resisting entry and multiplication of both exogenous and endogenous microbes. However, a weakened immune system makes it easier for these pathogens to invade the body and launch an infectious disease. This weakened state is referred to as immunodeficiency or immunocompromise.

Impaired function of white blood cells (WBCs), as well as low levels of T cells and B cells, characterizes immunodeficiencies. An immunodeficiency may be congenital or acquired. Acquired immunodeficiency may result from infection, malnutrition, chronic stress, or pregnancy. Diabetes, renal failure, and cirrhosis can suppress the immune response, as can such drugs as corticosteroids and chemotherapy.

Regardless of cause, the result of immunodeficiency is the same. The body’s ability to recognize and fight pathogens is impaired. People who are immunodeficient are more susceptible to all infections, are more acutely ill when they become infected, and require a much longer time period to heal.

Other conditions that may weaken a person’s immune defenses include poor hygiene, malnutrition, inadequate physical barriers, emotional and physical stressors, chronic diseases, medical and surgical treatments, and inadequate immunization.

Good hygiene promotes normal host defenses; poor hygiene increases the risk of infection. Unclean skin harbors microbes, offering an environment for them to colonize, and is more open to invasion. Frequent body washing removes surface microbes and maintains an intact barrier to infection, but it may damage the skin. To maintain skin integrity, lubricants and emollients may be used to prevent cracks and breaks.

The body requires a balanced diet to provide the nutrients, vitamins, and minerals needed for an effective immune system. Protein malnutrition inhibits the production of antibodies, without which the body is unable to mount an effective attack against microbe invasion. Malnutrition has been shown to have a direct relationship to the incidence of nosocomial infections.

Dust can facilitate transportation of pathogens. For example, dustborne spores of the fungus Aspergillus transmit the infection. If the inhaled spores become established in the lungs, they’re notoriously difficult to expel. Fortunately, most persons with intact immune systems can resist infection with Aspergillus, which is usually dangerous only in the presence of severe immunosuppression.

Extremely young and old people are at higher risk for infection. The immune system doesn’t fully develop until about age 6 months. An infant exposed to an infectious agent usually develops an infection. The most common type of infection in toddlers affects the respiratory tract. When young children put toys and other objects in their mouths, they increase their exposure to a variety of pathogens.

Exposure to communicable diseases continues throughout childhood, as children progress from daycare facilities to schools. Skin diseases, such as impetigo, and lice infestation commonly pass from one child to the next at this age. Accidents are common in childhood as well, and broken or abraded skin opens the way for bacterial invasion. Lack of immunization also contributes to incidence of childhood diseases.

Advancing age, on the other hand, is associated with a declining immune system, partly as a result of decreasing thymus function. Chronic diseases, such as diabetes and atherosclerosis, can weaken defenses by impairing blood flow and nutrient delivery to body systems.

A microbe must be present in sufficient quantities to cause a disease in a healthy human. The number needed to cause a disease varies from one microbe to the next and from host to host and may be affected by the mode of transmission. The severity of an infection depends on several factors, including the microbe’s pathogenicity, that is, the likelihood that it will cause pathogenic changes or disease. Factors that affect pathogenicity include: