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An infectious disease results from the invasion of a host by disease producing organisms, such as bacteria, viruses, fungi, or parasites. A communicable (or contagious) disease is one that can be transmitted from one source (i.e., person or animal) to another, by contact with the infected individual or bodily fluids, contact with contaminated surfaces or objects, or ingestion of contaminated food or water, or by contact with disease vectors such as mosquitos, fleas, or mice. Not all infectious diseases are communicable. For example, malaria and schistosomiasis are spread by contact with disease vectors. These are not typically considered to be communicable or contagious diseases because they cannot be spread by direct contact with an infected person. On the other hand, chickenpox is an infectious disease that is also highly communicable, because it can be easily transmitted from one person to another.

The mode of transmission is the mechanism by which an agent is transferred to the host. Modes of transmission include contact transmission (direct, indirect, droplet), airborne, vector‐borne, or common vehicle (food, equipment). Contact transmission is the most common mode of transmission in the EMS setting and can be effectively prevented using routine practices.

Direct contact transmission occurs when there is direct contact between an infected or colonized individual and a susceptible host. Transmission may occur, for example, by biting, kissing, or sexual contact. Indirect contact occurs when there is passive transfer of an infectious agent to a susceptible host through a contaminated intermediate object. This can occur if contaminated hands, equipment, or surfaces are not appropriately washed and decontaminated after patient contacts. Examples of diseases transmitted by direct or indirect contact include human immunodeficiency virus (HIV), hepatitis, and methicillin‐resistant Staphylococcus aureus (MRSA).

Droplet transmission is a form of contact transmission requiring special attention. It refers to large droplets generated from the respiratory tract of a patient when coughing or sneezing, or during invasive airway procedures such as intubation and suctioning. These droplets are propelled and may be deposited on the mucous membranes of the susceptible host. The droplets may also settle in the immediate environment, and the infectious agents may remain viable for prolonged periods, to be later transmitted by indirect contact. Examples of diseases transmitted by droplet transmission include meningitis, influenza, rhinovirus, respiratory syncytial virus, and coronavirus diseases.

Airborne transmission refers to the spread of infectious agents to susceptible hosts through the air. In this case, infectious agents are contained in very small droplets that can remain suspended in the air for prolonged periods. These agents are dispersed widely by air currents and can be inhaled by a susceptible host located at some distance from the source. Examples of airborne transmission diseases include measles (rubeola), varicella (chicken pox), and tuberculosis.

Vector‐borne transmission refers to the spread of infectious agents by means of an insect or animal (the “vector”). Examples of vector‐borne illnesses include rabies, where the infected animal is the vector, and West Nile virus or malaria, where infected mosquitos are the vectors. Transmission of vector‐borne illness does not occur between patients and EMS personnel.

Common vehicle transmission refers to the spread of infectious agents by a single contaminated source to multiple hosts. This can result in large outbreaks of disease. Examples of this type of transmission include contaminated water sources (Escherichia coli); contaminated food (Salmonella); or contaminated medication, medical equipment, or intravenous solutions.