(From Smolinski MS, Hamburg MA, Lederberg J, editors: Microbial threats to health: emergence, detection, and response, Washington, DC, 2003, National Academies Press.)

Box 20-1 Four Domains of Human-Microbe Interaction

Pathways in Prevention of Infectious Disease

1 Genetic and Biologic Factors

In human-microbe interaction, genetic and biologic factors include the makeup of the human body, with its physical, cellular, and molecular barriers to infection (human susceptibility to infection). Many of these factors are amenable to prevention efforts; exercise and a healthy diet contribute to intact barriers to infection. Biologic factors that increase infectivity of the microbe include its prevalence, stability, infectious dose, latency phase, and induction of shedding in the host. The noroviruses, the most common cause of diarrheal illness, exemplify the highly infectious microbe: viral particles are highly prevalent and can survive for a long time outside the human body; even a few viruses are enough to induce illness; and they are mainly transmitted from people who do not feel ill (viral shedding). Microbial adaptation and change also affects the interplay. Many microbes have successfully adapted to their environments through millions of years and continue to evolve. Their constant, rapid pace of mutation helps them develop resistance to potent antibiotics (e.g., vancomycin-intermediate staphylococcus aureus [VISA]) and complicates attempts to find vaccines (e.g., malaria, HIV).

Pathways to prevention in this domain include decreasing host susceptibility through better nutrition and vaccines, as well as constant vigilance for emerging resistance patterns.

2 Physical Environment Factors

Physical environment factors include the climate and latitude of an environment, which affect a location’s conduciveness to microbe or vector survival. Climate can directly impact disease transmission through replication and survival of pathogens and vectors, as well as through its effects on ecology. Landslides, earthquakes, and other natural disasters also create conditions conducive to the spread of infectious disease, such as overcrowding, lack of sanitation, and malnutrition.

Prevention efforts in this domain include improved food safety and sanitation, as well as focused surveillance on areas conducive to emerging infections.

3 Ecological Factors

Changes in ecosystems can effect the transmission of microbes through water, soil, air, food, or vectors. Such alterations also affect microbes with animal reservoirs. Examples include the changes of malaria prevalence in response to a warming climate and the increase in prevalence of Lyme disease because of more deer in expanding New England woods. Also important are changes in land use. A growing number of emerging infectious diseases arise from increased human contact with animal reservoirs (disruption/destabilization of natural habitats; see Chapter 30). An example is the Nipah virus, which was endemic to Southeast Asian fruit bats. When pig farms grew in size and density and expanded into fruit orchards in the late 1990s, the virus was transmitted to the pigs and then their handlers, causing encephalitis outbreaks. Pathways to prevention in this domain can be again found mainly through surveillance.

4 Social, Political, and Economic Factors

Human demographics and behavior involve international travel and commerce that can lead to rapid dissemination of infectious diseases (e.g., SARS) or produce-borne diseases. Advances in technology and industry open up new transmission modes (e.g., blood transfusion, use of antibiotics in farm animals). Furthermore, disruptions of peace and public health services as well as income inequality all worsen infectious diseases transmission. For example, war and famines are closely linked to the spread of infectious diseases, and mortality from infectious diseases is closely correlated with global poverty. Lack of political will has also contributed to delayed control. For example, the widespread perception in the second half of the 20th century was that infectious diseases were under control and no longer posed a public health threat. This complacency probably contributed to delays in detecting and controlling multidrug-resistant TB as well as food-borne outbreaks. A relatively new factor here is also the intent to harm through the release of microbial agents as an act of aggression.

Pathways for prevention through social, economic, and political factors lie in taking a comprehensive view of health, advocating for improvements to the underlying determinants in populations, and helping create the political will to strengthen public health and overcome health disparities (see Chapter 26).

Understanding and controlling infectious diseases requires integrating many different preventive and public health skills. These include obtaining accurate history on sensitive topics such as sexual behaviors; geographic epidemiology; outbreak investigation; analysis of disease rates by different variables (age, gender, race, socioeconomic status) to detect high-risk groups; successful outreach to public and health professionals; screening; contact tracing; immunization; school health; counseling; sanitation; waste and wastewater management; food protection; disease registries; and prophylactic drugs. Diseases vary, but the epidemiologic skills are similar for different diseases, independent of their mode of transmission (e.g., STD vs. vector-borne disease). Public health controls disease through prevention efforts in three broad categories, as follows ²:

Improving resistance of the host. Includes such basics as hygiene and nutrition, as well as vaccination, postexposure prophylaxis, and chemoprophylaxis (see Chapter 15).

Improving environmental safety. Includes sanitation, air quality control, water and food safety, and control of vectors and animal reservoirs. Public sanitation has been crucial in controlling infectious disease. Worldwide, areas without access to clean water and basic sanitation carry the highest burden of diseases that disproportionally impact children less than 5 years old.

Improving public health systems. Includes improved contact tracing, education, containment, and herd immunity.

All infection control activity requires a thorough understanding of the various infectious diseases. This chapter only briefly addresses the complexity of different diseases, and important diseases are discussed elsewhere (e.g., see Chapter 3 for influenza and Select Readings for further information). On the other hand, control of an infectious disease often only requires understanding how it is transmitted. For example, John Snow determined that water from a particular company caused most of the cholera in London. Armed with this understanding and the supporting data, he was able to convince the local council to disable the well. Breaking the chain of transmission helped end the outbreak.

Diseases can be usefully grouped according to transmission ³ (Table 20-1). Often, surveys of patients and “shoe leather” epidemiology will reveal the mode of transmission, and public health officials can disrupt disease transmission before the causative agent has been identified (see Chapter 3).

Table 20-1 Transmission of Infectious Diseases

A Burden of Disease

Infectious diseases affect all countries, but the burden of disease is different in developed and developing countries. In the United States, infectious disease mortality has for the most part steadily declined since the early 1900s.⁴ Most of this decline preceded the availability of antibiotics or vaccines and was likely the result of better hygiene, sanitation, and chlorination of drinking water.

Since the 1980s, the burden of infectious disease in the United States has again increased, largely because of emerging or reemerging infections, such as multidrug-resistant Staphylococcus aureus, Clostridium difficile, and Mycobacterium tuberculosis. Globally, infectious diseases account for about half the disease burden in low-income and middle-income countries. Infectious diseases especially impact children; more than half of childhood mortality is attributable to acute respiratory infections, measles, diarrheal illnesses, malaria, and human immunodeficiency virus and acquired immunodeficiency syndrome (HIV/AIDS).

To weigh the effects of disease on life span, many global health experts measure the impact of infectious disease in disability-adjusted life years (DALY). DALY take into account premature mortality and years of life lived in less than full health (see Chapter 24.) Five of the 10 leading diseases for global disease burden are infectious: HIV/AIDS, lower respiratory infections, diarrheal illnesses, malaria, and tuberculosis (TB). More importantly, many of the infectious diseases causing a large disease burden are increasing (HIV/AIDS, respiratory diseases) and are disproportionately impacting the lowest-income countries.⁵

See online Figure 20-2 on studentconsult.com for global mortality rates by cause and region.

Figure 20-2 Global death rates by disease group and region.
*Includes respiratory infections. Cause-specific estimated death rates for 1990 might not be completely comparable to those for 2001 because of changes in data availability and methods, plus some approximations in mapping 1990 estimates to the 2001 regions of East Asia and Pacific, South Asia, and Europe and Central Asia. For all geographic regions, high-income countries are excluded and shown as single group at top of graph. Therefore the “geographic regions” refer only to low-income and middle-income countries.

(From Lopez AD et al: Lancet 367:1747–1757, 2006.)

Worldwide death rates from malaria and HIV/AIDS are increasing. These increases have negated gains derived from reduced child mortality from measles, acute respiratory infections, and diarrhea.

B Obtaining an Accurate History

Transmission of major infectious diseases often results from a person’s behavior, including eating and hygiene habits, pets in residence, illicit drug use, and sexual partners. Therefore, caring for a patient with an infectious disease requires taking a careful behavioral history. The behaviors resulting in transmission can be mainstream and unrelated to any social taboos, such as the restaurant visited before a diarrheal illness.

More often, patients may be embarrassed by behavior that induced the infectious disease. Examples range from people kissing their pets (leading to transmission of Pasteurella spp.⁶) to sexual behaviors and use of illicit drugs (leading to transmission of sexually transmitted and blood-borne diseases). Patients may not be comfortable sharing such information unless the clinician is skilled at putting people at ease and asks about intimate details in a nonjudgmental way. Taking such a history is crucial for understanding how the patient contracted the infectious disease and who else may have been infected.

Client-centered counseling means tailoring prevention messages to a patient’s practices, values, and risk perceptions. For sexually transmitted diseases (STDs), or sexually transmitted infections (STIs), client-centered counseling has been shown to increase the likelihood of patients changing their behavior.⁷ The same likely holds true for other behaviors. As in other areas of counseling, it is important that the clinician start with open-ended questions and reassure the patient that the information will be treated confidentially.

Counseling for STDs is discussed in detail here for several reasons. STDs play a major role in infectious disease epidemiology, have a significant impact on fertility and pregnancy outcomes, and also may cause problems in newborns (e.g., syphilis, gonorrhea). Women are often more vulnerable to STDs than men, which poses particular challenges, because many effective interventions require male condoms. Also, effective counseling for STDs has been extensively researched, but many clinicians are uncomfortable addressing this topic.

Counseling for STDs can serve as a template for other sensitive topics, such as illicit drug use, risk taking among adolescents, and addiction. For STDs and other sensitive topics, it is particularly important that the interviewing techniques be culturally appropriate and, especially with adolescents, correspond to patients’ developmental levels. The interview should begin with more neutral topics in the social history (e.g., sports, activities, diet), then move to questions about sexual behaviors. In broaching such topics, it is important to frame the questions, as in the following examples:

For adolescents, “Now I am going to take a few minutes to ask you some sensitive questions that are important for me to help you be healthy. Anything we discuss will be completely confidential. I won’t discuss this with anyone, not even your parents, without your permission.”⁸ After clarifying this, introduce the topic in a nonthreatening way: “Some of my patients your age have started having sex. Have you had sex?”

For adults, “To provide the best care, I ask all my patients about their sexual activity. So, tell me about your sex life.”

Further history taking can follow the model of the “5 Ps” (partners, prevention of pregnancy, protection from STDs, practices, and past STDs).⁷

For each of those domains, again it is important to start with open-ended questions (e.g., “Tell me about how you have sex”; “Where do you meet your partners?”) before asking about specific high-risk behaviors.

Additional information on effective STD counseling and behavioral interventions can be found online.⁹

II Public Health Priorities

A HIV/AIDS, Tuberculosis, and Malaria

In some countries in sub-Saharan Africa, HIV/AIDS, tuberculosis, and malaria together account for more than 50% of deaths.¹⁰ These illnesses decrease health and constrain growth and development of many of the poorest nations. In general, they also impact developed countries, either internally through income inequality or externally through immigration and international travel. All these diseases have important lessons to offer for successful infectious disease prevention. Prevention efforts for these three diseases are often implemented together, as through the Global Fund to Fight AIDS, Tuberculosis, and Malaria.¹¹ The Global Fund follows an innovative model, targeting all three diseases through partnerships among government, civil society, the private sector (including businesses and foundations), and affected communities, combined with meticulous attention to data and evaluation.