KEY TERMS
Throughout history, until the beginning of the 20th century, infectious diseases were the major killers of humans. Bubonic plague, the “Black Death,” is said to have wiped out as much as 75 percent of the population of Europe and Asia in the 14th century. Tuberculosis was the number one killer in England in the mid-19th century. An example of the toll of infectious diseases is demonstrated in (FIGURE 9-1), which provides death rates of the population of New York City over the period 1804 to 2013. Epidemics of smallpox and cholera swept through the city every few years, killing many people in each wave. In the mid-19th century, background mortality rates—largely from tuberculosis, typhoid, and miscellaneous respiratory and gastrointestinal diseases—were double what they became by 1930.
These infectious diseases were largely conquered through public health measures, including purification of water, proper disposal of sewage, pasteurization of milk, and immunization, as well as improved nutrition and personal hygiene. The discovery and introduction of antibiotics in the 1940s also played a role. In fact, by the 1960s, the threat of infectious diseases seemed to have been reduced to a minor nuisance.
In contrast to the fear, drama, and excitement that accompanied efforts to understand and control infectious diseases in the late 19th and early 20th centuries, public health in the 1960s and 1970s seemed to have become routine and boring. This period in the history of public health corresponds to the time when, according to the Institute of Medicine, public health was falling into disarray because of complacency.1 This chapter will focus on the battles public health practitioners have won. It will discuss the causes of infectious diseases, how they are transmitted, and how classic public health measures have brought them under control.
FIGURE 9-1 Death Rates in New York City, 1804–2013
Reproduced from Zimmerman, R. et al. Summary of Vital Statistics, 2013: Mortality. New York, NY: New York City Department of Health and Mental Hygiene, Office of Vital Statistics, 2015.
Infectious Agents
The major epidemic diseases are caused by bacteria, viruses, or parasites. The fact that each of these diseases is caused by a specific microbe was established in the 1880s and 1890s, at a time of great scientific excitement, when almost every year marked a discovery of a new disease-causing bacterium.
Robert Koch, a German physician, developed techniques to classify bacteria by their shape and their propensity to be stained by various dyes. Since billions of bacteria—most of them harmless to humans—inhabit the skin, throat, mouth, nose, large intestine, and vagina, it was necessary to develop a set of rules that could be used to prove that a specific organism caused a specific disease. These rules, called “Koch’s postulates,” are (1) the organism must be present in every case of the disease; (2) the organism must be isolated and grown in the laboratory; (3) when injected with the laboratory-grown culture, susceptible test animals must develop the disease; and (4) the organism must be isolated from the newly infected animals and the process repeated.2
Koch applied these rules in his proof that tubercle bacilli were the cause of tuberculosis, the leading cause of death in Europe at that time. Bacilli are bacteria that appear rod-shaped when observed under the microscope. Koch identified another bacillus, Vibrio cholera, as the cause of cholera. Other disease-causing bacilli identified during that period were those that cause plague, typhoid, tetanus, diphtheria, and dysentery.
Bacilli, Cocci, Spirochete.
Roundworm, Hookworm, Pinworm, Tapeworm.
Round-shaped bacteria, called cocci, include streptococci, which cause strep throat and scarlet fever; staphylococci, which cause wound infections; and pneumococci, which cause pneumonia. Syphilis is caused by a corkscrew-shaped bacterium called a spirochete. All these bacteria were identified by the beginning of the 20th century.
For some infectious diseases, however, no bacterial agent could be found. Smallpox, for example, was known to be transmitted from a sick person to a healthy one by something in the pus of the patient’s lesions. Yet attempts to isolate a microorganism were unsuccessful. The agent that caused the disease could pass through the finest available filters and could not be observed in any existing microscope. Smallpox was recognized to be one of a number of diseases caused by such “filterable agents” or viruses. It was not until 1935, when the American scientist W. M. Stanley crystallized tobacco mosaic virus, that the nature of viruses was demonstrated.
While bacteria are living, single-celled organisms that can grow and reproduce outside the body if given the appropriate nutrients, viruses are not complete cells. They are simply complexes of nucleic acid and protein that lack the machinery to reproduce themselves. Various kinds of viruses infect not only animal cells but also plant cells—as tobacco mosaic virus infects tobacco—and even bacteria. They can survive extreme conditions such as treatment with alcohol and drying in a vacuum and become active again when they are injected into a living cell. They reproduce themselves by taking control of the cell’s machinery, often killing the cell in the process. The human diseases caused by viruses include smallpox, yellow fever, polio, hepatitis, influenza, measles, rabies, and AIDS, as well as the common cold.
Human diseases can also be caused by protozoa, or single-celled animals that can live as parasites in the human body. Malaria, spread by mosquitoes; cryptospiridiosis, which caused the Milwaukee diarrhea epidemic described earlier in this text; and giardiasis, also known as “beaver fever” are examples of protozoal diseases. Other parasites, such as roundworms, tapeworms, hookworms, and pinworms, are the most common source of human infection in the world. Except for pinworms, they are not common in the United States today.
Means of Transmission
Infectious diseases are spread by a variety of routes, directly from one person to another or indirectly by way of water, food, or vectors such as insects and animals. Bacteria and viruses that cause respiratory infections, including colds, influenza, and tuberculosis, are transmitted through the air on aerosols, water droplets produced when an infected person coughs or sneezes. They can also be transmitted from an infected person to objects he or she touches, such as doorknobs, utensils, or towels, to be picked up by the next person to touch the contaminated object and transferred by hand to the nose. The early European settlers made use of this route of transmission to inflict a primitive form of germ warfare on the Native American people, giving them blankets that had been used by patients suffering from smallpox. The disease decimated Native American populations because they had no immunity to the virus.
Gastrointestinal infections such as cholera, cryptospiridiosis, and diphtheria are generally spread by the fecal–oral route, by which fecal matter from an infected person reaches the mouth of an uninfected person. This may occur as a result of poor personal hygiene or by contamination of drinking water because of inadequate sanitary systems. Vector-borne diseases, including malaria, yellow fever, and West Nile encephalitis, generally use a more complex route from one person to another, most often through an insect.
Each disease has its own pattern of development after a person is infected, and the time during which the patient is capable of transmitting the infection to others varies from one disease to another. Some diseases are most likely to be transmitted during the most symptomatic phase, for example, when a patient suffering from tuberculosis or the common cold is most actively coughing and sneezing. Others, such as measles and mumps, are most communicable during the day or two before noticeable symptoms develop. A few diseases can exist in a carrier state, in which the infected person can transmit the disease without having symptoms, as demonstrated by the infamous case of Typhoid Mary.3
Mary Mallon worked as a cook in a series of wealthy New York homes at the beginning of the 20th century. After an increasing number of family members in these homes became sick with typhoid fever, some of them fatally, suspicion fell on the cook. Because she was healthy, and because cooking was the only way she knew to support herself, Mary resisted medical tests and, when finally proven to be a carrier of the bacteria, refused to accept the results. Eventually she had to be incarcerated to prevent her from taking jobs where she spread the disease by the fecal-oral route. She remained in the custody of the New York City Health Department for the rest of her life. It was Mary’s occupation, of course, that made her such a threat to the public health. The discovery of antibiotics, too late to help Mary, made it possible to eliminate the bacteria in typhoid carriers. However some viruses, such as herpes and hepatitis B, can persist in carrier states, and no treatment is known to eliminate them.
Chain of Infection
Control of infectious diseases is still an important component of public health. The public health approach to controlling infectious diseases is to interrupt the chain of infection. Many methods used to accomplish this interruption have now become routine, but vigilance is always required.
The chain of infection, a term used to describe the pattern by which an infectious disease is transmitted from person to person, is composed of several links, as illustrated in (FIGURE 9-2). These are listed here:
1. Pathogen. The pathogen is a virus, bacterium, or parasite that causes the disease in humans.
2. Reservoir. The reservoir is a place where the pathogen lives and multiplies. Some pathogens spread directly from one human to another and have no other reservoir. Others, however, may infect nonhuman species, spreading from them to humans only occasionally. Plague, for example, is a disease of rodents that is transmitted to humans by the bite of a flea. Rats are the reservoir of plague. Raccoons and bats are reservoirs for rabies, which spreads to humans only through the bite of a rabid animal. Contaminated water or food may also serve as reservoirs for some human diseases.
FIGURE 9-2 Chain of Infection
3. Method of transmission. The pathogen must have a way to travel from one host to another, or from a reservoir to a new host. The flea is a vector for plague, transferring the plague bacillus from rat to human by sucking it up when it bites the rat and then injecting it into a human host with a second bite. Food-borne diseases are transmitted when a person eats contaminated food; water-borne diseases are transmitted when someone drinks contaminated water. Many respiratory diseases are transmitted by aerosol. AIDS, syphilis, gonorrhea, and a number of other diseases are transmitted by sexual contact.
4. Susceptible host. Even if the pathogen gains entry, a new potential host may not be susceptible because the host has immunity to the pathogen. Immunity may develop as a result of previous exposure to the pathogen, or the host may naturally lack susceptibility for a variety of reasons. Most microorganisms are specifically adapted to infect certain species. Canine distemper virus, for example, does not infect humans. Even within species, susceptibility to specific viruses varies among individuals. Scientists have been puzzled why a very few people who have been repeatedly exposed to the human immunodeficiency virus (HIV) do not become infected; recent studies have found a genetic mutation that makes them resistant to the virus.
Public health measures to control the spread of disease are aimed at interrupting the chain of infection at whichever links are most vulnerable. At link 1, the pathogen could be killed, for example, by using an antibiotic to destroy the disease-causing bacteria. At link 2, one could eliminate a reservoir that harbors the pathogen. For example, controlling rat populations in cities by picking up garbage is a way of preventing the spread of plague to humans. Adequate water and sewage treatment prevents the spread of water-borne diseases, and proper food-handling methods eliminate reservoirs of food-borne pathogens.
At link 3, transmission from one host to another could be prevented by quarantining infected individuals, for example, or by warning people to boil their water if the water supply becomes contaminated. Hand washing is an important way to prevent the spread of disease: it prevents restaurant workers from contaminating food, hospital workers from carrying pathogens from one patient to another, and allows all individuals to protect themselves against pathogens they may pick up from the environment and put in their mouth. The spread of sexually transmitted diseases can be prevented by use of a condom, a simple matter of blocking the movement of the pathogens to the uninfected person.
At link 4, the resistance of hosts can be increased by immunization, which stimulates the body’s immune system to recognize the pathogen and to attack it during any future exposure. Vaccination not only keeps the individual from contracting a disease but also makes it harder for the pathogen to find susceptible hosts. In some cases, it may even be possible to completely eliminate a pathogen from the earth by eliminating the susceptibility of its potential hosts. This was accomplished in the case of smallpox, as discussed below.
Other links are often included separately as part of the chain of infection when it is useful to consider them as sites for public health intervention. For example, the port of entry into the host for a mosquito-borne disease would be the skin, a link that could be interrupted if the potential host wears long sleeves and gloves. Similarly, the place of exit is the route by which the pathogen leaves the host.
Public health measures to control the spread of infectious disease include both routine prevention measures and emergency measures to control an outbreak once it has begun. Many of the measures referred to above—especially those concerning links 2 and 3—come under the category of “environmental health.” Immunization—link 4—is a major weapon that has had great success against the dread diseases that created the epidemics of the past. However, vaccines do not exist for all diseases—notably, a vaccine has not yet been developed against AIDS. Even when vaccines do exist, some diseases are too rare to justify the trouble and expense of vaccinating everyone. This is where surveillance is especially important.
Epidemiologic surveillance is the system by which public health practitioners watch for disease threats so that they may step in and break the chain of infection, halting the spread of disease. In the early history of public health, the solution was often quarantine—isolation of the patient to prevent him or her from infecting others. Quarantine is still used occasionally, when the disease is serious and there is no effective vaccine. For example, a patient diagnosed with tuberculosis—which is slow to respond to medication—might be ordered to stay home for 2 to 4 weeks after treatment is started until the disease is no longer infectious.
More often, the public health response when an outbreak is detected by surveillance is to locate people who have had contact with the infected individual and to immunize them or give them medical treatment, as appropriate. For tuberculosis, contact tracing is used in addition to quarantine: people who have been exposed to the patient are given prophylactic doses of antibiotics. Tuberculosis has presented new and more difficult problems to the public health system in recent years because of the development of drug-resistant strains of the bacteria.
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