Nurses are responsible for preventing infections in their clients and themselves. Infection control is vital for health care organizations. An understanding of the infectious process is important for nurses. Pathogenicity is the ability of a microorganism to produce disease. Virulence is the degree of pathogenicity of an infection’s microorganism. Infection is an invasion and multiplication of microorganisms (infectious agents) in body tissue, resulting in cellular injury. Communicable agents are microorganisms that can be transmitted to a client by direct or indirect contact, through a vehicle or vector or through the airborne route. Communicable agents cause communicable diseases.

Colonization is the multiplication of microorganisms on or within a host that does not result in cellular injury. These microorganisms may later cause an infection. Flora are the vegetation of microorganisms on the human body. Resident flora are always present and transient flora are episodic.

The infection chain is a process necessary for an infection to occur. The first link of the infection chain is the microorganism, or infectious agent. Infectious agents, or microorganisms, include bacteria, viruses, fungi, protozoa, and rickettsia. Their ability to cause an infection depends upon their virulence, number present, and ability to enter and live in the client, as well as on the susceptibility of the client. Resident infectious agents are always present on the skin; transient infectious agents are picked up by the skin from another person or object.

The second link in the infection chain is the reservoir, a place for the organism to live while awaiting a host. Common reservoirs are the human body, food, plants, animals, and feces. Microorganisms need food, oxygen, a viable temperature range, water, a pH of 5 to 8, and lack of sunlight. The third link in the infection chain is the portal of exit. Microorganisms leave the reservoir through direct contact, indirect contact, and airborne transmission. Direct contact, the most common portal of exit; it includes touching, kissing, and sexual intercourse. Indirect contact, the transmission of a communicable disease by any medium, includes vehicles and vectors. A vehicle, or fomite, is an object that has become contaminated with a microorganism. A vector is usually an insect. Airborne transmission occurs when an infected person coughs, thereby spreading infectious agents through the air on dust particles.

The fourth link in the infection chain is the mode of transmission, the process between the portal of exit from the reservoir to the portal entry of the new host. The most frequent is contact transmission, but airborne, vehicle or vector borne transmissions also occur. The fifth link in the infection chain is the portal of entry, the way by which an infectious agent gains access to the body. Examples are breaks in the natural skin barrier or through inhalation. The final link in the infection chain is the susceptible host. A host is an organism that can be affected by an agent. A susceptible host is a person who lacks resistance to an agent and is vulnerable to disease. A compromised host is a person whose normal defense mechanisms are impaired.

The interaction between agent and host occurs in the environment. Environmental factors affecting the infection chain are water, food, plants, animals, housing conditions, noise, meteorological conditions, and environmental chemicals. Conditions that promote the transmission of microorganisms are anthropogenic. The host factors influencing a microorganism’s capability to produce an infectious disease are the host’s age, hereditary background, stress level, nutritional status, and general health status (medical conditions and surgeries). The elderly are more susceptible to nosocomial (hospital-acquired) infections. Hereditary factors include deficiencies in serum antibodies or inability to form immunoglobulins. Stress affects the normal immune response, while adequate nutrition provides the proteins necessary for wound healing. Medical problems and surgeries affect the immune system.

The host’s normal defense mechanisms depend upon the ability of the immune system’s to recognize which antigens (agents) are consistent with the genetic composition of the host and which are not. Immune responses are nonspecific and specific responses. Nonspecific immune defenses act against all microorganisms. Nonspecific immune defenses are skin and normal flora; mucous membranes; sneeze, cough, and tearing reflexes; an acidic environment; and the inflammatory response. Inflammation is a nonspecific cellular response to tissue injury or infection. Tissue injury is caused by bacteria, trauma, chemicals, heat, and other factors. When the tissue is injured, (1) several chemicals (histamine, bradykinin, serotonin, prostaglandins, and lymphokines) are released; (2) blood is shunted to the injured area; (3) the capillaries become more permeable, resulting in release of plasma into the damaged tissue; (4) leukocytes are released, producing exudates (pus); and (5) destroyed tissues are replaced with healthy tissue and scars.

Specific immune responses are categorized as immunity. When antigens (foreign particles) enter the body, macrophages, monocytes, or neutrophils digest or phagocytize them. Some antigens, autoantigens, originate from the body’s own proteins. Antibodies are formed by the immune system in response to antigens. There are several types of immune responses. Active immunity results when antibodies are formed to neutralize or destroy an infective agent. Passive immunity is acquired by the introduction of preformed antibodies, such as the passing of antibodies from mother to fetus in utero. Acquired immunity results from exposure to an antigen or from the passive injection of immunoglobulins. Natural immunity is the genetically determined development of antibodies within a specific species. Artificial immunity is produced following a vaccine.

When antigens are processed, they are released to lymphocytes, stimulating humoral immunity (the production of B lymphocytes) or cell-mediated immunity (the production of T lymphocytes). The humoral immune response is initiated when an antigen related to a bacteria or virus is recognized and attacked by a macrophage. The macrophage process the antigen by attaching a B cell, resulting in the production of large amounts of antibodies, or immunoglobulins. Cell-activated immunity is initiated when the antigen stimulates the release of activated T cells. Cellular immunity helps fight infections within the cells.

The two types of infectious responses are localized infections, confined to a single area or organ, and systemic infections, affecting the entire body. The first stage of infection is the incubation period, the time between entry of an infectious agent in the host and the onset of symptoms during which the client is infectious to others. The second stage of infection is the prodromal stage, the time interval between the onset of nonspecific symptoms and the manifestation of specific symptoms. The client may be infectious during this period. The third stage of infection is the illness stage, when the client manifests symptoms of the disease. The fourth and final stage of infection is the convalescent stage, in which the acute symptoms disappear and the client returns to the previous state of health.

Nosocomial infections are those that are contracted in a health care setting. Nosocomial infections are present in 5% to 15% of the clients admitted to hospitals in the United States, but they are preventable. Preventive measures are avoiding urinary catheterization, thorough and frequent handwashing or use of alcohol-based hand rubs, and using clean equipment.

Asepsis is the absence of microorganisms. Aseptic technique is the use of infection control to prevent the transmission of pathogens. There are two types of aseptic technique. Medical asepsis incorporates practices used to reduce the number, growth, and spread of microorganisms. Objects are considered “clean” (without pathogenic microorganisms) or “dirty” (soiled, with the possibility of multiple potentially pathogenic microorganisms). Medical asepsis includes handwashing, gloving, changing linens daily, and cleaning floors and hospital furniture daily. The Centers for Disease Control recommends vigorous scrubbing of hands with warm, soapy water for at least 15 seconds to prevent the transfer of germs.

Surgical asepsis is sterile technique, consisting of practices that eliminate all microorganisms and spores (resting or inactive microorganisms) from an object or area. Objects can be decontaminated through the use of one or all of three processes. Cleansing is the removal of soil or organic material from instruments and equipment used in providing client care. Disinfection is the elimination of pathogens, except spores, from inanimate objects, using disinfectants, which are chemical solutions used to clean inanimate objects. Germicides, chemicals that can be applied to both animate and inanimate objects to eliminate pathogens, may also be used. Sterilization is the total elimination of all microorganisms, including spores.

All medical waste is considered infectious and is disposed of carefully, using gloves, proper containers, labeling of waste, and handwashing. Standard precautions (isolation precautions) have been developed to prevent the spread of infections in health care facilities, based on whether the infection is spread through contact, droplets, or airborne transmission.