OUTCOME OF INFECTIOUS DISEASES
Given the complexities of host defenses and microbial virulence, it is not surprising that the factors determining outcome between these two living entities are also com plicated. Basically, outcome depends on the state of the host’s health, the virulence of the pathogen, and whether the host can clear the pathogen before infection and disease cause irreparable harm or death (Figure 1).
Fig1. Possible outcomes of infections and infectious diseases.
The time from exposure to an infectious agent and the development of a disease or infection depends on host and microbial factors. Infectious processes that develop quickly are referred to as acute infections, and those that develop and progress slowly, sometimes over a period of years, are known as chronic infections. Some pathogens, particularly certain viruses, can be clinically silent inside the body without any noticeable effect on the host before suddenly causing a severe and acute infection. During the silent phase, the infection is said to be latent. Again, depending on host and microbial factors, acute, chronic, or latent infections can result in any of the outcomes detailed in Figure 1.
Medical intervention can help the host fight the infection but usually is not instituted until after the host is aware that an infectious process is underway. The clues that an infection is occurring are known as the signs and symptoms of disease and result from host responses (e.g., inflammatory and immune responses) to the action of microbial virulence factors (Box 1). Signs are measurable indications or physical observations, such as an increase in body temperature (fever) or the development of a rash or swelling. Symptoms are indictors as described by the patient, such as headache, aches, fatigue, and nausea. The signs and symptoms reflect the stages of infection. In turn, the stages of infection generally reflect the stages in host-microorganism interactions (Figure 2).
Box1. Signs and Symptoms of Infection and Infectious Diseases
Fig2. Host-microorganism interactions and stages of infection or disease.
Whether medical procedures contribute to control ling or clearing an infection depends on key factors, including:
• The severity of the infection, which is determined by host and microbial interactions already discussed
• Accuracy in diagnosing the pathogen or pathogens causing the infection
• Whether the patient receives appropriate treatment for the infection (which depends on accurate diagnosis)
PREVENTION OF INFECTIOUS DISEASES
The treatment of an infection is often difficult and not always successful. Because much of the damage may already have been done before appropriate medical intervention is provided, the microorganisms gain too much of a “head start.” Another strategy for combating infectious diseases is to stop infections before they start (i.e., disease prevention). As discussed at the beginning of this chapter, the first step in any host-microorganism relationship is the encounter and exposure to the infectious agent. Therefore, strategies to prevent disease involve interrupting or minimizing the risk of infection when exposures occur. As outlined in Box 2, interruption of encounters may be accomplished by pre venting transmission of the infecting agents and by controlling or destroying reservoirs of human pathogens. Interestingly, most of these measures do not really involve medical practices but rather social practices and policies.
Box2. Strategies for Preventing Infectious Diseases
Immunization
Medical strategies exist for minimizing the risk of disease development when exposure to infectious agents occurs. One of the most effective methods is vaccination, also referred to as immunization. This practice takes advantage of the specificity and memory of the immune system. The two basic approaches to immunization are active immunization and passive immunization. With active immunization, modified antigens from pathogenic microorganisms are introduced into the body and cause an immune response. If or when the host encounters the pathogen in nature, the memory of the immune system ensures minimal delay in the immune response, thus affording strong protection. With passive immunization, antibodies against a particular pathogen that have been produced in one host are transferred to a second host, where they provide temporary protection. The passage of maternal antibodies to the newborn is a key example of natural passive immunization. Active immunity is generally longer lasting, because the immunized host’s own immune response has been activated. However, for complex reasons, naturally acquired active immunity has had limited success for relatively few infectious diseases, necessitating the development of vaccines. Successful immunization has proven effective against many infectious diseases, including diphtheria, whooping cough (pertussis), tetanus, influenza, polio, smallpox, measles, hepatitis, and certain Streptococcus pneumoniae and Haemophilus influenzae infections.
Prophylactic antimicrobial therapy, the administration of antibiotics when the risk of developing an infection is high, is another common medical intervention for pre venting infection.
Epidemiology
To prevent infectious diseases, information is required regarding the sources of pathogens, the mode of trans mission to and among humans, human risk factors for encountering the pathogen and developing infection, and factors that contribute to good and bad outcomes resulting from the exposure. Epidemiology is the science that characterizes these aspects of infectious diseases and monitors the effect diseases have on public health. Fully characterizing the circumstances associated with the acquisition and dissemination of infectious diseases gives researchers a better chance of preventing and eliminating these diseases. Additionally, many epidemiologic strategies developed for use in public health systems also apply in long-term care facilities (i.e., nursing homes, hospitals, assisted living centers) for the control of infections acquired within the facility (i.e., nosocomial infections; for more information on infection control).
The field of epidemiology is broad and complex. Diagnostic microbiology laboratory personnel and epidemiologists often work closely to investigate problems. Therefore, familiarity with certain epidemiologic terms and concepts is important.
Because the central focus of epidemiology is on tracking and characterizing infections and infectious diseases, this field heavily depends on diagnostic microbiology. Epidemiologic investigations cannot proceed unless researchers first know the etiologic or causative agents. Therefore, the procedures and protocols used in diagnostic microbiology to detect, isolate, and characterize human pathogens are essential for patient care and also play a central role in epidemiologic studies focused on disease prevention and the general improvement of public health. In fact, microbiologists who work in clinical laboratories are often the first to recognize patterns that suggest potential outbreaks or epidemics.
