Additional Material for Study, Review, and Further Exploration

Throughout history, epidemics have been responsible for the destruction of entire groups of people. Despite amazing advances in public health and health care, control of communicable diseases continues to be a major concern of health care providers. The emergence of new pathogens, the reemergence of old pathogens, and the appearance of drug-resistant pathogens are creating formidable challenges worldwide.

Multicausation

Spectrum of Infection

Not all contact with an infectious agent leads to infection, and not all infection leads to an infectious disease. The processes, however, begin in the same way. An infectious agent may contaminate the skin or mucous membranes of a host, but not invade the host. Or it may invade, multiply, and produce a subclinical infection (inapparent or asymptomatic) without producing overt symptomatic disease. Or the host may respond with overt symptomatic infectious disease. Infection, then, is the entry and multiplication of an infectious agent in a host. Infectious disease and communicable disease refer to the pathophysiological responses of the host to the infectious agent, manifesting as an illness. Such a person, when the disease is diagnosed in him or her, would be considered a case. Once infectious agents replicate in a host, they can be transmitted from the host irrespective of the presence of disease symptoms. Some persons become carriers and continue to shed the infectious agent without any symptoms of the disease.

Spectrum of Disease Occurrence

Infectious Agents

Because the process of transmission is different for every infectious agent, one might envision a different configuration of the chain and its links for each infectious agent and infectious disease that exists. Infectious agents act differently, depending on their intrinsic properties and interaction with their human host. For example, an agent’s size, shape, chemical composition, growth requirements, and viability (ability to survive for extended periods of time) have an impact on transmission and the type of parasitic relationship it establishes with its host. These characteristics determine the classifications of different agents (e.g., prions, viruses, bacteria, fungi, and protozoa), and knowing the classification is helpful in understanding how specific agents are transmitted and produce disease. Also important are how the agent interacts with its host and its mode of action in the body. For example, Mycobacterium tuberculosis kills cells, but the spirochete that causes syphilis interferes with circulation. Or maybe it produces a toxin (toxigenicity) as does Clostridium botulinum, or stimulates an immune response in the host (antigenicity), as does rubella virus. Other considerations for understanding the action of agents include their power to invade and infect large numbers of people (infectivity), their ability to produce disease in those infected with the agent (pathogenicity), and their ability to produce serious disease in their hosts (virulence). Applying the above concepts, the chickenpox virus has high infectivity, high pathogenicity, and very low virulence. On the other hand, M. tuberculosis has low infectivity, low pathogenicity, but high virulence if untreated. Smallpox virus is high on all three concepts. Last, one must consider how adaptable an agent is to its human host and whether the agent changes, or mutates, over time, as HIV does.

Reservoirs

The environment in which a pathogen lives and multiplies is the reservoir. Reservoirs can be humans, animals, arthropods, plants, soil, water or any other organic substance. Some agents have more than one reservoir. Knowing the reservoirs for infectious agents is important because, in some cases, transmission can be controlled by eliminating the reservoir, such as eliminating the standing water where mosquitos breed.

Portals of Exit and Entry

Agents leave the human host through a portal of exit and invade through a portal of entry. Portals of exit include respiratory secretions, vaginal secretions, semen, saliva, lesion exudates, blood, and feces. Portals of entry are associated with the portal of exit and include the respiratory passages, mucous membranes, skin and blood vessels, oral cavity, and the placenta.

Modes of Transmission

Direct transmission is the immediate transfer of an infectious agent from an infected host or reservoir to an appropriate portal of entry in the human host through physical contact, such as a touch, bite, kiss, or sexual contact. Direct projections of mucous secretions by droplet spray to the conjunctiva, or mucous membranes of the eye, nose, or mouth during coughing, sneezing, and laughing is also considered direct transmission. Direct person-to-person contact is responsible for the transmission of many communicable diseases (e.g., STDs, influenza).

Indirect transmission is the spread of infection through a vehicle of transmission outside the host. These may be contaminated fomites or vectors. Fomites can be any inanimate objects, materials, or substances that act as transport agents for a microbe (e.g., water, a telephone, or a contaminated tissue). The infectious agent may or may not reproduce on or in the fomite. Substances such as food, water, and blood products can provide indirect transmission through ingestion and intravenous transfusions. Botulism is an example of an indirectly transmitted food-borne enterotoxin disease.

Vectors can be animals or arthropods, and they can transmit through biological and mechanical routes. The mechanical route involves no multiplication or growth of the parasite or microbe within the animal or vector itself. Such is the case when a housefly carries gastrointestinal agents from raw sewage to uncovered food. Biological transmission occurs when the parasite grows or multiplies inside the animal, vector, or arthropod. Examples of diseases spread by this method of transmission include arthropod-borne diseases such as malaria, hemorrhagic fevers, and viral encephalitis. Transmission from a vector to the human host is usually through a bite or sting. Such is the case with the mosquito vector that transmits St. Louis encephalitis and West Nile virus.

Fecal-oral transmission can be direct and indirect. It can occur indirectly through the ingestion of water that has been fecally polluted or by consumption of contaminated food. Direct transmission occurs through engagement in oral sexual activity. Poliovirus and hepatitis A are spread through fecal-oral routes.

Airborne transmission occurs mainly through aerosols and droplet nuclei. The timeframe in which an airborne particle can remain suspended greatly influences the virility and infectivity of the organism. The size of the particle can also determine how long it remains airborne and how successful it will be at penetrating the human lung. Aerosols are extremely small solid or liquid particles that may include fungal spores, viruses, and bacteria. Droplet nuclei, such as the spray from sneezing or coughing, may make direct contact with an open wound or with a mucous membrane, or they may be inhaled into the lung. TB is spread through inhalation of contaminated droplets.

Host Susceptibility

Not all humans are equally susceptible or at risk for contracting an infection or developing an infectious disease. Biological and personal characteristics play an important role. Just as the young are at greater risk for diphtheria, older adults are at greater risk for bacterial pneumonia. General health status is important, as evidenced by the increased risk for gastrointestinal parasites in children living in poverty. Personal behaviors certainly influence susceptibility, as does the presence of healthy lines of defense. The immune system and immunization status play important roles in the increased number of infections in unimmunized and immunocompromised persons.

Controlling the Agent

Controlling the agent is an area where technology and medical science have been extremely effective. Inactivating an agent is the principle behind disinfection, sterilization, and radiation of fomites that may harbor pathogens. Anti-infective drugs, such as antibiotics, antivirals, antiretrovirals, and antimalarials, play important roles in controlling infectious diseases. Not only do they permit recovery of the infected person, but they also play a major role in preventing transmission of the pathogens to another. The first step in preventing transmission of tuberculosis and syphilis is to treat the infected person with antibiotics.

Eradicating the Nonhuman Reservoir

Common nonhuman reservoirs for pathogens in the environment include water, food, milk, animals, insects, and sewage. Treating or eliminating them is an effective method of preventing replication of pathogens, and thus preventing transmission.

Controlling the Human Reservoir

Treating infected persons, whether they are symptomatic or not, is effective in preventing transmission of pathogens directly to others. A quarantine is an enforced isolation or restriction of movement of those who have been exposed to an infectious agent during the incubation period; this is another method of controlling the reservoir. Quarantine was used effectively during the outbreak of severe acute respiratory syndrome (SARS) in 2003, when some hospitals required that their staff exposed to SARS patients remain at the hospital until proved to be symptom free at the end of the incubation period. Quarantine was selectively implemented during the H1N1 outbreak in 2009-2010.

Controlling the Portals of Exit and Entry

The transmission chain may be broken at the portal of exit by properly disposing of secretions, excretions, and exudates from infected persons. Additionally, isolation of sick persons from others and requiring that persons with tuberculosis wear a mask in public can be effective.

The portal of entry of pathogens also can be controlled by using barrier precautions (masks, gloves, condoms); avoiding unnecessary invasive procedures, such as indwelling catheters; and protecting oneself from vectors, such as mosquitoes. In response to the risk of exposure to blood-borne pathogens (e.g., HIV, hepatitis B, and hepatitis C) in the late 1980s, the CDC developed a set of guidelines, called universal precautions, to prevent transmission of diseases found in blood and other body fluids. These guidelines were developed because infected people may be asymptomatic and have no knowledge of their conditions; therefore health care workers must assume that all patients are infectious and protect themselves.

Improving Host Resistance and Immunity

When administered according to established guidelines and protocols, vaccines provide acquired immunity in most cases. However, there are exceptions. Primary vaccine failure is the failure of a vaccine to stimulate any immune response. It can be caused by improper storage that may render the vaccines ineffective, improper administration route, or exposure of light-sensitive vaccines to light. Additionally, some immunized persons never seroconvert, either because of failure of their own immune system or for some other unknown reason. Secondary vaccine failure is the waning of immunity following an initial immune response. This often occurs in immunosuppressed patients and organ transplant patients in whom the immune memory is essentially destroyed.

Terminology: Control, Elimination, and Eradication