A communicable disease is an illness caused by a specific infectious agent or its toxic products. It arises through transmission of that agent or its products from an infected person, animal, or inanimate reservoir to a susceptible host, either directly or indirectly (through an intermediate plant or animal host, vector, or the inanimate environment). Control of disease is the reduction of disease incidence, prevalence, morbidity, or mortality to a locally acceptable level as a result of deliberate efforts; continued intervention measures are required to maintain the reduction. Control is to be contrasted with elimination (reduction to zero of the incidence of a specified disease in a defined geographic area as a result of deliberate efforts; continued intervention measures are required), eradication (permanent reduction to zero of the worldwide incidence of infection caused by a specific agent as a result of deliberate efforts; intervention measures are no longer needed), and extinction (the specific infectious agent no longer exists in nature or the laboratory).
Communicable diseases may be classified according to the causative agent, the clinical illness caused, or the means of transmission. Often all three characteristics are used (e.g., foodborne Salmonella gastroenteritis). Causative agents include bacteria, viruses, and parasites. Examples of bacterial diseases include pneumococcal pneumonia and gonorrhea. Viral diseases include influenza, measles, and ebola. Parasitic diseases include malaria and schistosomiasis. Other communicable diseases may be caused by other types of microorganisms such as fungi (e.g., histoplasmosis). The types of illness include pneumonia, diarrhea, meningitis, or other clinical syndromes.
Various categorizations of means of transmission have been used. The American Public Health Association uses these categories: direct transmission, indirect transmission, and airborne. Direct transmission refers to direct contact such as touching, biting, kissing, or sexual intercourse, or the direct projection of droplet spray into the eye, nose, or mouth during sneezing, coughing, spitting, singing, or talking. This projection usually is limited to a distance of 1 meter or less. Examples of direct contact transmission include rabies and sexually transmitted HIV (human immunodeficiency virus). Direct projection is responsible for transmission of diseases such as measles and influenza.
Indirect transmission may occur through a vehicle or an arthropod vector. The causative agent may or may not multiply or develop in or on the vehicle. Examples of possible vehicles include water, food, biological products, or contaminated articles (such as syringe needles). Water-and food-borne diseases have the potential for causing outbreaks involving thousands of persons. Before the causative agent was identified, many cases of HIV resulted from blood transfusion. Since all donor blood in the United States is now screened for HIV, this is no longer a significant means of transmission. However, sharing of needles by injection drug users remains an important factor in the AIDS (acquired immunodeficiency syndrome) epidemic. Arthropod vectors can spread disease mechanically (as a result of contamination of their feet or passage of organisms through the gastrointestinal tract) or biologically (in which the agent must multiply or go through one or more stages of its life cycle before the arthropod becomes infective). Mechanical spread by arthropod vectors is uncommon. However, arthropod-borne diseases such as malaria (in which the parasite develops within the mosquito vector) are still responsible for millions of cases and hundreds of thousands of deaths each year in tropical countries.
Some infectious agents can be spread through the air over long distances. Airborne spread requires that infectious particles are small enough to be suspended in the air and inhaled by the recipient. Tuberculosis and histoplasmosis are bacterial and fungal diseases spread in this fashion. Airborne transmission could also be used to disseminate agents of biological warfare or bioterrorism. Anthrax and smallpox have been considered among the most likely biological weapons.
Diseases of animals that can be spread to humans are called zoonoses. Some zoonotic diseases include rabies, plague, and tularemia (rabbit fever).
Methods of Communicable Disease Control
Communicable diseases occur only when the causative agent comes into contact with a susceptible host in a suitable environment. Prevention and control efforts for communicable diseases may be directed to any of these three elements. Communicable diseases affect both individuals and communities, so control efforts may be directed at both. Treatment of persons with communicable diseases with antibiotics typically kills the agent and renders them noninfectious. Thus, treatment is also prevention. A simple way to prevent the occurrence of communicable diseases is to eliminate the infectious agent through, for example, cooking food, washing hands, and sterilizing surgical instruments between use. Assuring the safety of drinking water through filtration and chlorination and treating sewage appropriately are other important means of preventing the spread of communicable diseases.
For most communicable diseases there is an interval between infection and occurrence of symptoms (the incubation period) in which the infectious agent is multiplying or developing. Some persons who are infected may never develop manifestations of the disease even though they may be capable of transmitting it (inapparent infection). Some persons may carry (and transmit) the agent over prolonged periods (carriers) whether or not they develop symptoms. Treatment during the incubation period may cure the infection, thereby preventing both disease and transmission. This preventive treatment (chemoprophylaxis) is often used in persons who have been exposed to sexually transmitted diseases such as syphilis and gonorrhea. It also is effective in persons who have been infected with tuberculosis, although the preventive treatment must be given for several months.
The susceptibility of the host to a specific infectious agent can be altered through immunization (e.g., against measles) or through taking medications that can prevent establishment of infection following exposure (chemoprophylaxis). Since malnutrition and specific vitamin deficiencies (such as vitamin A) may increase susceptibility to infection, ensuring proper nutrition and administering vitamin A can be more general ways of increasing host resistance. If persons survive a communicable disease, he or she may develop immunity that will prevent the disease from recurring if re-exposed to the causative agent.
The environment may be rendered less suitable for the occurrence of disease in a variety of ways. For example, food can be kept hot or cold (rather than warm) to prevent multiplication of organisms that may be present. Individuals can use mosquito repellents or mosquito nets to prevent being bitten by infected mosquitoes. Breeding places can be drained or insecticides used to eliminate vectors of disease. Condoms can be used to prevent sexually transmitted diseases by providing a mechanical barrier to transmission. Reduction of crowding and appropriate ventilation can reduce the likelihood of droplet or airborne transmission. Respiratory protective devices can be used to prevent passage of microorganisms into the respiratory tract.
The sociocultural environment is also important in affecting the occurrence of communicable diseases. For example, in the 1980s there was a change in the social norms in men who have sex with other men on the West Coast of the United States, where unprotected anal intercourse had been the norm and was responsible for considerable transmission of HIV. As a result of a variety of educational and social marketing approaches, the social norm changed to the use of condoms and the rate of new HIV infections (and of rectal gonorrhea) declined. Similarly, aggressive social marketing of condom use in Uganda has led to a change in sexual practices and a decline in new HIV infection rates. Other societal approaches to control of communicable diseases include safe water and food laws, provision of free immunization and chemoprophylaxis through public health departments, enactment and enforcement of school immunization requirements, isolation of individuals with communicable diseases to prevent transmission, and quarantine of individuals exposed to communicable diseases to prevent disease transmission during the incubation period if they have been infected.
Impact of Communicable Diseases
The gathering of humans in settlements (and subsequently cities) resulted in the development of periodic epidemics of communicable diseases, often with devastating impact. In the fourteenth century, for example, bubonic plague (carried by rats and transmitted to humans by fleas) swept through Europe, killing approximately one-quarter of the population of the continent. Epidemics of “crowd” diseases such as measles and influenza resulted from person-to-person transmission, and inadequate water and sewage management led to epidemics of diseases such as cholera and typhoid. Milk- and food-borne diseases also were common. Until the end of the nineteenth century, communicable diseases were the leading cause of death throughout the world.
In the United States in 1900, tuberculosis was the leading cause of death, followed by pneumonia and diarrhea. Along with diphtheria (in tenth place), these conditions accounted for more than 30 percent of all deaths in the country. Major reductions in morbidity and mortality from communicable diseases have resulted from improvements in sanitation, housing, and nutrition as well as introduction and use of vaccines and specific therapies.
Improvements in sanitation have dramatically reduced the burden of water- and food-borne diseases. Improvements in housing have also played an important role in reducing transmission of tuberculosis, and improvements in nutrition have made persons with infectious diseases less likely to die from their infections. The introduction and use of vaccines have resulted in global eradication of smallpox, significant progress toward eradication of poliomyelitis, and a marked reduction in illness and death due to diseases such as diphtheria, whooping cough (pertussis), and measles. Specific therapies such as antibiotics and antiparasitic drugs have had a significant impact on deaths due to infectious diseases as well as having some impact on the occurrence of the diseases by shortening the period in which an infected person is infectious to others.
The most dramatic improvements have been seen in the United States and other developed nations (see Figure 1). Although significant progress has also been made in developing nations, the World Health Report 2000 reports that 14 million deaths (25 percent of all deaths in the world in 1999) resulted from infectious diseases or their complications. There is a marked disparity in the importance of infectious diseases in high-income countries compared to middle- and low-income countries. In high-income countries, infectious diseases accounted for only 6 percent of all deaths, whereas in middle- and low-income countries they accounted for 28 percent of all deaths.
Worldwide, lower respiratory infections (e.g., pneumonia) and diarrhea are the leading infectious causes of death; each of these conditions can be caused by a variety of microorganisms. AIDS was the single leading infectious cause of death in 1998, with an estimated 2.2 million deaths, followed by tuberculosis, with nearly 1.5 million deaths, and malaria, with 1.1 million deaths. Nearly 900,000 children died as a result of measles in 1998, even though an effective vaccine against measles was introduced in 1963 and has had a major impact in developed nations. Half of the children who died from measles lived in sub-Saharan Africa.
Much of the continuing toll of communicable diseases could be reduced by more effective use of existing vaccines and other tools for control of infectious diseases. For example, more effective use of measles vaccine and administration of vitamin A could prevent most of the deaths from measles. More widespread use of oral rehydration therapy in diarrhea (to combat the dehydration that is one of the major causes of death) could dramatically reduce current mortality. More effective use of bed nets, anti-mosquito strategies, and appropriate treatment could dramatically reduce malaria deaths. However, new tools will be needed to bring about maximum control of some diseases. Because microorganisms are continually evolving, they may change enough so that prior experience (infection) with the infectious agent does not provide protection. For example, influenza viruses may undergo dramatic changes with the result that pandemics (worldwide epidemics) may occur. In 1918–1919, pandemic influenza killed millions of people worldwide, more than 500,000 in the United States alone (see Figure 1).
Some communicable diseases can be prevented by the use of vaccines. The word vaccine comes from vaccinia, the Latin name for cowpox. The first vaccine was developed by Edward Jenner, an eighteenth-century English physician and naturalist who noticed that milkmaids who had acquired cowpox (a condition that caused lesions to appear on the udders of cows) on their hands did not seem to be affected by smallpox. He believed that infection with cowpox would protect against smallpox, a serious, often fatal epidemic disease. In 1796 he took material from a skin lesion on the hand of a milkmaid and inoculated it into the arm of a young boy. The boy was subsequently exposed to smallpox and did not become ill. Thus began the vaccine era.
It was nearly one hundred years until the next vaccine (rabies) was developed by Louis Pasteur. In the twentieth century, a number of vaccines were developed; many more are under development as a result of the biotechnology revolution. Widespread use of vaccines in children has had a dramatic impact on the occurrence of the diseases.
Because smallpox has been eradicated, smallpox vaccination is no longer carried out. The last case of naturally occurring smallpox in a human was in 1977, and in 1980 the World Health Assembly certified that smallpox had been eradicated from the face of the earth. Stocks of smallpox virus have been maintained (under security) in both the United States and Russia, though the debate continues whether they should be destroyed. Concerns have arisen about the possibility that some groups or nations have retained the smallpox virus and developed it for use in biological warfare or bioterrorism.
Chemoprophylaxis refers to the practice of giving anti-infective drugs to prevent occurrence of disease in individuals who are likely to be exposed to an infectious disease or who might have already been infected but have not developed disease. For example, individuals traveling to areas where malaria is common can take anti-malarial drugs before arriving, during their stay, and for a few weeks after leaving and thus protect themselves against malaria. Similarly, persons who have been exposed to syphilis can be given penicillin to prevent the possibility of their developing syphilis, and persons who have been infected with tuberculosis can be given six months of treatment to prevent the development of tuberculosis.
Antibiotics and Resistance
Antibiotics are compounds that are produced by microorganisms that kill or inhibit the growth of other microorganisms. Those that kill bacteria are called bactericidal; those that prevent multiplication (and rely on the body’s defense mechanisms to deal with the limited number of living organisms) are called bacteriostatic. Some antibiotics are effective against a limited number of microorganisms, others may have more widespread effect.
Because microorganisms are continually in a state of evolution, strains may evolve that are resistant to a particular antibiotic. In addition, resistance characteristics can be transferred from some microorganisms to others (this is particularly true of organisms that inhabit the gastrointestinal tract). The likelihood that resistance will develop is increased if antibiotics are used in an indiscriminate manner and in inadequate amounts (either in terms of individual dosage or in length of therapy). Antimicrobial resistance is a growing problem: organisms that once were exquisitely sensitive to a particular antibiotic may now have developed significant (or total) resistance to it. This necessitates either increasing the dose of the antibiotic administered (in the case of partial resistance) or developing totally new drugs to treat the infection (in the case of total resistance). A few microorganisms (such as enterococcus, an organism that lives in the intestinal tract and is particularly likely to cause infections in gravely ill patients with compromised immune systems) have developed such widespread resistance that it is a real challenge to treat them effectively, resulting in a need to develop even more antibiotics.
Emerging and Re-Emerging Infectious Diseases
New infectious diseases continue to be recognized and others, once thought under control, are reemerging as significant problems. To cite a few examples of “new” diseases, the following have been recognized for the first time since 1975: legionnaire’s disease, ebola virus, HIV/AIDS (acquired immunodeficiency syndrome), toxic shock syndrome, Escherichia coli O157:H7 (cause of hemolytic-uremic syndrome), Lyme disease, Helicobacter pylori (major cause of peptic ulcer), hepatitis C, and hantavirus. Some of these are conditions previously known but without a known infectious cause (e.g., peptic ulcer) while others represent apparently new clinical syndromes that have not occurred or have not been recognized in the past.
Old diseases, such as tuberculosis and malaria, are reemerging in areas where they were once under control. This may be a result of the lack of continued application of known effective interventions but also may result from ecological changes. Some of the factors involved in the increase in infectious diseases, whether new or old, include population shifts and growth (and encroachment on previously unpopulated areas); changes in behavior (e.g., injection drug use, sexual practices); urbanization, poverty, and crowding; changes in ecology and climate; evolution of microbes; inadequacy of the public health infrastructure to deal with the problems; modern travel and trade; and the increasing numbers of persons with compromised immune systems (whether as a result of HIV/AIDS, chemotherapy for cancer, or immunosuppresive therapy for organ transplants). Many of these factors are interrelated.
In addition to these new and reemerging diseases, there may be specific interactions between diseases. This is particularly true with HIV and tuberculosis (TB), in which each infection is a very potent co-factor for worsening the other: Persons with HIV infection who become infected with TB are more likely to develop TB disease that is serious and rapidly progressive than persons without HIV infection, and persons with TB who contract HIV infection are very likely to have a rapid progression to full-blown AIDS.
In the United States, the incidence of foodborne disease has received increasing attention in the past several years. This relates in part to improved surveillance but also relates to changes in patterns of food production, distribution, and consumption. With modern transportation, it is possible to get fresh vegetables and fruits at all times of the year. This means that salad ingredients purchased at a modern supermarket (and eaten raw) may have been grown in a developing country, where the average American traveler would not eat raw vegetables. The consolidation of producers of prepared foods makes possible large interstate outbreaks of food-borne disease such as the 1994 outbreak of Salmonella infections associated with ice cream that affected an estimated 224,000 persons nationwide. It is currently estimated that food-borne diseases cause approximately 76 million illnesses, 325,000 hospitalizations, and 5,000 deaths in the United States each year.
Epidemic Theory and Mathematical Models of Infectious Diseases
Based on observed characteristics of infectious diseases, epidemiologists have attempted to construct mathematical models that would make it possible to predict the pattern of spread of a condition within the population. Some diseases have constant features, which make mathematical modeling particularly attractive. Measles, for example, has a predictable incubation period (ten to fourteen days) and limited duration of infectivity of a given patient (four to seven days). In addition, it is highly infectious (nearly every susceptible person who comes in contact with an infectious person will become infected), and nearly everyone who is infected develops clinical illness. Lifelong immunity follows infection. There is no nonhuman reservoir. Given these relatively constant parameters, it is possible to predict the pattern of transmission if measles is introduced into a population, using different estimates for the proportion of susceptible persons in the population, the distribution of these susceptibles (e.g., randomly dispersed, clustered together), and the likelihood of contact between the infectious patient and the susceptibles. Because of the extreme infectiousness of measles, models indicate that it is necessary to reach very high levels of immunity in a population (on the order of 95 percent or greater) in order to prevent sustained transmission of measles. Given the fact that measles vaccine is approximately 95 percent effective, this indicates that, to eradicate measles, it will be necessary to reach 100 percent of the population with a single dose of the vaccine or to reach 90 percent of the population on each of two rounds of vaccination (assuming that the second round will reach 90 percent of those who were not reached by the first round). Since babies are being born all the time, this also must be an ongoing process. The major reason for continuing debate over whether measles eradication is an achievable goal using current vaccines is the necessity to achieve and maintain such high levels of immunity.
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