Cause and Spread of Infection

Introduction

In this section, you'll learn about how infection can spread. Specifically, you'll review the chain of infection and how medical professionals look for a way to break the chain.

 

The Chain of Infection

For disease to spread, there are six steps that need to occur. These steps are called the chain of infection. Healthcare professionals are always looking for a way to break the chain of infection, thereby stopping the spread of diseases.

The chain of infections includes:

1. Infectious Agents

2. Reservoir

3. Portal of Exit

4. Means of Transmission

5. Portal of Entry

6. Susceptible Host

 

Any microorganism capable of causing disease is considered to be a pathogen, or infectious agent. This is the first link in the chain of infection. In order for disease to occur, the pathogen must be present. There are five classifications of pathogenic microorganisms: viruses, bacteria, fungi, parasites, or rickettsiae. 

Viruses are pathogens that need a living cell of a host to be active and reproduce. They're considered to be intracellular parasites because they actually have to be inside the target cell to multiply and cause disease. Viruses are very good at mutating and adapting to their environment, making them very hard to cure. There are only a few pharmacologic agents that are used to treat viruses. Most are considered to be palliative because they treat and reduce only the symptoms of the viral disease instead of eliminating the disease itself.

The term palliative describes a treatment or medicine that reduces the effects or symptoms of a medical condition without curing it.

Bacteria are single-celled microorganisms that live in the body's tissues. Many bacteria are able to survive and reproduce outside the host organism. They're identified and classified by their characteristic shapes and morphology. There are three classifications based on bacterial shape: cocci (shaped like a sphere or a dot), bacilli (rod-shaped), and spirilla (spiral-shaped).

Bacteria can also be grouped by how they stain in the laboratory setting. Gram-negative bacteria become visibly red under the microscope when stained. Gram-positive bacteria appear violet under the microscope. If the bacteria doesn't accept the bacterial stain at all, they're considered to be spores. Spores have a covering (like a hard shell) that protects them, making it very challenging to kill this type of bacteria with chemical disinfectants or heat. Humans have nonpathogenic bacteria that reside on the skin and in the mucous membrane areas of the body. These nonpathogenic bacteria are called normal flora. This flora is necessary and important to help us maintain our body's homeostasis and minimize the growth of pathogens.

Fungi may be single- or multicelled. Mushrooms and mold are nonpathogenic. Pathogenic fungi cause athlete's foot, ringworm, and Candida infections.

Any organism that lives in or on another organism is classified as a parasite. These organism may be single- or multicelled. A protozoan is a single-celled parasite. Examples of protozoa are malaria and dysentery. Metazoans are multicelled parasites that include pinworms, tapeworms, and hookworms.

Rickettsiae are intracellular, parasitic, nonmoving bacteria. They're a little larger than the typical virus. These types of bacteria are susceptible to antibiotics. Examples of rickettsial infections include typhus, Lyme disease, and Rocky Mountain spotted fever. These infections often have a characteristic rash caused by the rickettsia invading the small blood vessels.

 

The reservoir is the second link in the chain of infection. This is the place where the pathogen lives and grows as it waits for the next opportunity to spread. People, animals, insects, water, and the food supply are all examples of potential reservoirs. Methods for breaking the chain of infection at this second link include hand washing, disinfection, and sterilization. 

 

The portal of exit is the third link in the chain of infection. There needs to be a way for the pathogen to leave the reservoir and spread.

Pathogens usually leave via secretions or excretions such as mucus, vomit, blood, urine, feces, vaginal fluid, and semen.

 

Now, if the pathogen has a portal of exit, then it needs a way to travel to the next reservoir to continue to spread. The fourth link in the chain of infection is means of transmission.

Pathogens can be transmitted in several different ways, as shown.

Direct Contact

A person directly touches the pathogen or comes in direct contact with a body fluid.

Airborne Transmission

A pathogen is in the air and it's inhaled. The common cold and tuberculosis are two examples of pathogens that can be transmitted through the air.

Bloodborne Transmission

Infected blood or body fluids enter a susceptible host.

Ingestion 

Pathogens are transmitted through eating or drinking from contaminated sources.

Indirect Contact Transmission

Pathogens enter when interaction with a fomite occurs. Remember, fomites are inanimate objects and surfaces that we can touch. Door knobs, remote controls, and keyboards are all examples of fomites. How long a pathogen can be on a fomite and still cause infection depends on each individual pathogen, and to some extent whether the fomite is a porous or nonporous surface.

Vector

Animals or insects that carry disease easily transmit pathogens. Mosquitoes can carry the West Nile virus and deer ticks can carry Lyme disease.

 

Even if a pathogen can travel, it still needs a way to get into a person. The fifth link in the chain of infection is the portal of entry. 

Pathogens can enter through breaks in the skin, mucous membranes, inhalation and ingestion, and sexual intercourse.

To break the chain of infection at the portal of entry, healthcare workers employ methods of infection control such as correct wound care, transmission-based precautions, and aseptic technique.

 

For a person to become infected with a pathogen, he or she must be a susceptible host, the sixth and final link in the chain of infection. A susceptible host is someone who is able to contract the pathogen. Vaccines can help break this link in the chain of infection, therefore making a person not susceptible. Susceptibility of a person depends on several factors:

• The number and species of pathogen
• The duration of exposure to the pathogen
• The host's overall general physical condition
• The host's psychological health status
• The host's occupational or lifestyle environment
• Age of the host
• The presence of underlying diseases or conditions in the host

 

 

Nosocomial Infections

Nosocomial infections are infections that are acquired while patients are in the hospital. These infections can be caused by pathogens present on the healthcare staff's hands, equipment, or instruments.

Nosocomial infections are preventable and should not occur.

Studies have demonstrated that clostridium difficile, which is a spore-forming bacterium, can remain intact on surfaces such as the hospital curtains and TV remote controls for an extended period of time. That's why disinfecting and sanitizing hospital rooms and equipment is very important.

 

Key Points

• For a disease to spread, the six steps of the chain of infection need to occur:                                                                                                                         1. An infectious agent must be present.                                                                                                                                                                                           2. The reservoir allows the pathogen to live and grow.                                                                                                                                                                 3. The portal of exit allows the pathogen to leave the reservoir.                                                                                                                                                 4. The means of transmission allows the pathogen to spread.                                                                                                                                                   5. The portal of entry allows the pathogen to leave the body.                                                                                                                                                     6. The susceptible host is someone who then contracts the pathogen.
• Nosocomial infections are acquired while patients are in the hospital.