Chain of Infection

19.2 ■  Chain of Infection Learning Outcome Describe the steps involved in the chain of infection. The spread of an infectious disease follows a series of steps often termed the chain of infection. First, there must be a source, or reservoir, of an infectious agent. If the reservoir is an infected host, the agent must leave that host through a portal of exit, be transmitted to a new host, and then enter the new host through a portal of entry ( figure 19.2 ). Knowing the chain of infection for a given disease enables researchers and public health workers to determine where links in the chain can be broken, thereby stopping or slowing the spread of the disease. A flowchart shows the chain of infection. large image navigator opens in a modal FIGURE 19.2 Chain of Infection  If any link in this chain is broken, disease transmission is slowed or stopped. Access the text alternative for Figure 19.2 How could disease transmission be stopped at the portal of exit? Reservoirs of Infection The natural habitat of a pathogen, the reservoir of infection , may be on or in an animal (including humans) or in an environment such as soil or water ( figure 19.3 ). The reservoir of infection affects the extent and distribution of a disease. Once the reservoir is identified, susceptible people can be prevented from coming into contact with the disease source.  An illustration shows the reservoirs of infection. large image navigator opens in a modal FIGURE 19.3 Reservoirs of Infection (top): Thinkstock/Getty Images; (bottom): Rudmer Zwerver/Shutterstock Access the text alternative for Figure 19.3 How might you be a reservoir of infection? Human Reservoirs Infected humans are a significant reservoir of most communicable diseases. Humans are oft... ... released into the air during a cough generally cause disease only when someone inhales them; for these pathogens, the nose is the typical portal of entry. As described for droplet transmission, a properly worn face mask can help prevent inhalation of pathogen-containing respiratory particles. Intestinal pathogens such as Vibrio cholerae and  Shigella species must usually be ingested; the mouth is the portal of entry. For  Shigella species, the infectious dose is about 10 to 100 cells, a number easily transferred by casual contact; if cells of a Shigella species were transferred by touch, however, they would cause disease only if they were then transported to the mouth. When fecal organisms are inadvertently ingested, the transfer is called fecal-oral transmission . Shigella Some organisms can cause disease if they enter one body site but are harmless if they enter another. For instance, Enterococcus faecalis , a member of the normal intestinal microbiota, can cause a bladder infection if it enters the urinary tract, but it is harmless when ingested. Page 486 MicroAssessment 19.2 Spread of a disease can be prevented by breaking the chain of infection. The reservoir of a disease agent can be infected people, other animals, or the environment. To spread, infectious microbes must exit one reservoir, be transmitted to a susceptible host, and then enter that host. Handwashing and vector control can prevent many diseases; airborne transmission of pathogens is difficult to control. How can an arthropod act as both a mechanical vector and a biological vector? How does handwashing reduce the spread of disease? Considering that circulating blood is not normally released from the body, describe how blood-borne microbes might exit.