12 THE ADA PRACTICAL GUIDE TO EFFECTIVE INFECTION CONTROL The Infectious Disease Process: the Chain of Infection The process by which infectious diseases are transmitted involves three essential components, commonly referred to as the chain of infection: • A causative agent • A susceptible host • A mode of transmission All three components are necessary for an infection to spread. Effective infection control strategies are intended to break one or more “links” in the chain, thereby preventing disease transmission. Causative Agent A causative agent is any microorganism capable of causing disease. These microorganisms are referred to as pathogens. Pathogenic agents include a variety of viruses, bacteria, protozoa and fungi. Pathogens may be present in human blood (bloodborne pathogens) and other potentially infectious materials (OPIM), such as saliva in dental procedures. The hepatitis B virus (HBV), the hepatitis C virus (HCV), and the human immunodeficiency virus (HIV) — the agents of most concern to dental personnel — are bloodborne pathogens. Susceptible Host A susceptible host is a person who lacks effective resistance to a particular pathogenic agent. Many factors, including heredity, nutritional status, use of medications, therapeutic procedures, underlying disease and immunization status, influence a person’s level of susceptibility to a particular agent, as well as the severity of disease resulting from an infection. For example, a person with inadequate nutrition may be more susceptible to infection and may have a more serious disease than someone with adequate nutrition. Medications, such as steroids, and therapeutic procedures, such as chemotherapy, are also known to increase susceptibility. Underlying diseases, such as diabetes, increase the risk for and severity of infection. Finally, immunization status also influences susceptibility. People can reduce or eliminate their susceptibility to many diseases, such as polio, influenza and hepatitis B, through vaccination against them. Immunity to future infection is also acquired for some diseases, including hepatitis B, chickenpox and measles, if a person has had the disease and has developed antibodies against it. Modes of Transmission A mode of transmission is the mechanism by which an infectious agent is transferred to a susceptible host. Most infectious agents are transferred by direct or indirect contact and through inhalation of organisms in the air. Contact transmission may occur through direct contact with infectious body fluids, such as blood into an open wound. It may occur through indirect contact, for example, through a contaminated intermediate object such as a dental instrument or work surface. Finally, it may occur through contact with airborne contaminants present in either droplet splatter or aerosols. Disease transmission via droplet splatter may occur when an infected person sneezes or coughs, or when spatter is produced during dental procedures, and these fluids directly come into contact with an open wound or mucous membranes (e.g., eye). Disease transmission via aerosols, on the other hand, occurs through the inhalation of minuscule “infectious nuclei” that remain suspended in the air for extended periods of time. These “infectious Effective infection control strategies are intended to break one or more “links” in the chain of infection, thereby preventing disease transmission.