Covid transmission

How diseases are transmitted

To reduce the risk of harmful microorganisms, you must understand how different HAIs can spread. Microorganisms are found in air, soil, water, and in our bodies. Some microorganisms are helpful, others can be harmful and cause infections.

In a healthcare setting, the chain of infection refers to the numerous opportunities for pathogens to spread among patients, HCP, and the environment. Effective IPC aims to break the chain.

Infectious Agent

An infectious agent or microorganism, such as a virus, bacteria, or other microbe, that is capable of causing infection.

Reservoir

The reservoir is where microorganisms can be found. This can be a person (patient or HCP) or the environment. The healthcare environment contains diverse microorganisms. Environmental reservoirs include dry surfaces (bed rails and medical equipment), wet surfaces (faucets, sinks, and ventilators), indwelling medical devices (catheters and IV lines), and the environment around the patient.

Portal of Exit

Microorganisms must exit their reservoir to spread. For example, when someone coughs, microorganisms leave the reservoir (the person) through the respiratory tract. Portals of exit can include breaks in skin, mucous membranes (eyes, nose, and mouth), hands, blood, and the gastrointestinal and urinary tracts (as feces, vomit, and urine). Portals of exit can also be splashes from emesis (vomiting) patients or an exposed needle in an open sharps container.

Mode of Transmission

Microorganisms need a way to move (spread) from the portal of exit to the portal of entry. In other words, they need a way to get from point A to point B. Microorganisms usually depend on people, the environment, and medical equipment to move in healthcare settings.

Modes of transmission include:

• Direct
• Direct contact
• Droplet spread
• Indirect
• Airborne
• Contaminated equipment
• In direct transmission, an infectious agent is transferred from a reservoir to a susceptible host by direct contact or droplet spread.Direct contact refers mainly to skin-to-skin contact, but it can also refer to contact with soiled surfaces.
• Droplet spread refers to spray with relatively large, short-range aerosols produced by sneezing, coughing, or even talking.

Indirect transmission refers to the transfer of an infectious agent from a reservoir to a host via suspended air particles and inanimate objects or equipment.

• Airborne transmission refers to infectious agents being carried over long distances through the air. Airborne particles include material that has settled on surfaces and become resuspended by air currents, as well as infectious particles blown from the soil by the wind. In contrast to droplets that fall to the ground within a few feet, airborne infectious agents can remain suspended in the air for long periods of time and can be blown over great distances.
• Contaminated items that can indirectly transmit infectious agents include food, water, biological products (blood), and fomites (inanimate objects such as bedding, surgical instruments, shared equipment such as x-ray and dialysis equipment).

Portal of Entry

IV catheters (and other devices) and surgical incisions can provide entryways for microorganisms to gain access to a susceptible host.

Mucous membranes (eyes, nose, and mouth) are an entryway for microorganisms spread by direct contact, sprays, and splashes. When healthcare personnel wear masks or face shields, this prevents microorganisms from coming in contact with the eyes, nose, or mouth.

Breaks in the skin, such as a puncture caused by a sharps injury, can also be an entryway. Notice that portals of entry can also serve as portals of exit and reservoirs for harmful microorganisms.

Susceptible Host

The final link in the chain is the susceptible host. When patients receive medical treatment, the following factors can increase susceptibility to infection:

• Patients who have underlying medical conditions such as diabetes, cancer, and organ transplantation are at increased risk for infection. These illnesses often decrease the immune system’s ability to fight infection.
• Certain medications, such as antibiotics, steroids, and some chemotherapy medications, increase the risk of some types of infections.
• Medical devices and procedures, such as urinary catheters, tubes, and surgery, increase risk of infection by providing additional ways for microorganisms to enter the body.

Understanding the chain of infections—infectious agents, reservoirs, modes of transmission, portals of entry and exit, and susceptible hosts—enables us to develop IPC measures to break the chain of infection.