Elements of Infection

ELEMENTS OF INFECTION

Causative Organism

The types of microorganisms that cause infections are bacteria, rickettsiae, viruses, protozoa, fungi, and helminths.

Reservoir

Reservoir is the term used for any person, plant, animal, substance, or location that provides nourishment for microorganisms and enables further dispersal of the organism. Infections may be prevented by eliminating the causative organisms from the reservoir.

Mode of Exit

The organism must have a mode of exit from a reservoir. An infected host must shed organisms to another or to the environment before transmission can occur. Organisms exit through the respiratory tract, the gastrointestinal tract, the genitourinary tract, and the blood.

Route of Transmission

A route of transmission is necessary to connect the infectious source with its new host. Organisms may be transmitted through sexual contact, skin-to-skin contact, percutaneous injection, or infectious particles carried in the air. A person who carries, or transmits, an organism and who does not have apparent signs and symptoms of infection is called a carrier.

It is important to recognize that different organisms require specific routes of transmission for infection to occur. For example, Mycobacterium tuberculosis is almost always transmitted by theairborne route. Health care providers do not “carry” M. tubercu-losis bacteria on their hands or clothing. In contrast, bacteria suchas Staphylococcus aureus are easily transmitted from patient to patient on the hands of health care providers.

When appropriate, the nurse should explain routes of disease transmission to patients. For example, a nurse may explain that sharing a room with a patient who is infected with human immuno-deficiency virus (HIV) does not pose a risk because intimate con-tact (ie, sexual or parenteral) is necessary for transmission to occur.

Susceptible Host

For infection to occur, the host must be susceptible (ie, not pos-sessing immunity to a particular pathogen). Previous infection or vaccine administration may render the host immune (ie, not susceptible) to further infection with an agent. Many infections are pre-vented because of the powerful human immune defense. Although exposure to potentially infectious microorganisms occurs essentially on a constant basis, our elaborate immune systems generally pre-vent infection from occurring. The immune-suppressed person has much greater susceptibility than the normal, healthy host.

Portal of Entry

A portal of entry is needed for the organism to gain access to the host. For example, airborne M. tuberculosis does not cause disease when it settles on the skin of an exposed host. The only entry route for the bacterium that is of concern is through the respira-tory system.