Contagious Diseases

Death in early childhood was a heartbreaking fact of life everywhere until the early twentieth century. Gravestones from the nineteenth century and earlier commemorate the death before the end of their first decade of life of between a quarter and half of all the children born into most families. Nearly all died of contagious diseases.

Definition of Contagious Disease

Contagious means infectious by direct contact, but in common usage the word usually applies to diseases that are highly infectious and dangerous, implying something more serious than a common cold (although colds are among the most contagious of all diseases). In his 1548 De Contagione the Italian monk Girolamo Fracastorius identified three modes of contagion: by direct intimate contact (as with sexually transmitted diseases); by "droplets" (as in coughing, sneezing, and talking); and by contaminated articles (such as clothing and kitchen utensils). Other modes of contagion that Fracastorius did not recognize include transmission by polluted water or contaminated milk or food and transmission by insect vectors such as mosquitoes.

The Nature, Origin, Ecology, and Evolution of Contagious Diseases

The agents, called pathogens, that cause contagious diseases vary in size and complexity from ultramicroscopic viruses, microscopic bacteria, and single-celled protozoa like the malaria parasite, to parasitic worms up to several meters long. The survival of pathogens depends on their ability to invade humans or other hosts, in which they are nourished and reproduce, and from which they can spread to new hosts. This capability is determined by biological, ecological, and behavioral factors, all of which render children especially vulnerable, as discussed below. Pathogens flourish at the expense of the health, and perhaps the life, of the human host. The most successful pathogens do not kill their hosts but merely make them ill while producing symptoms conducive to the pathogens' spread to fresh hosts. The viruses that cause the common cold are particularly successful in this respect.

When the world warmed up after the last Ice Age (i.e., about 10,000 years ago), human ancestors discovered how to domesticate animals for food, milk, and clothing, and how to plant and harvest crops. These discoveries made possible the transition from nomadic hunter-gatherers to settled agriculturalists, and transformed human communities forever. Brief generation times and prodigious reproductive rates enabled some microorganisms that had previously affected only animals to evolve into human pathogens, aided by the increasingly close and frequent contacts between humans and their domesticated animals. In this way some bacteria and viruses became human pathogens for the first time. This probably was how measles, smallpox, influenza, and tuberculosis originated as contagious human diseases. Some important pathogens developed survival mechanisms involving humans and other creatures–dogs, sheep, cattle, pigs, mosquitoes, ticks, freshwater snails, etc. The relationship of humans and mosquitoes to water set the stage for the evolutionary origin of malaria, which has caused enormous misery and innumerable deaths, mostly of children, since before the dawn of history. Similar ecological connections between humans, water, and various other living creatures led to the evolution of schistosomiasis and parasitic infestations with several kinds of worms. Archeological discoveries have revealed evidence that some of the great scourges of human-kind–including smallpox, tuberculosis, malaria, and schistosomiasis–date from neolithic times, 10,000 to 15,000 years ago. Other diseases, including measles, influenza, typhus, and yellow fever, arose more recently, and the process continues. In the final quarter of the twentieth century, more than thirty new contagious diseases emerged and the pathogens that cause them were identified. Many of the newly emergent diseases are deadly–including HIV/AIDS, Lassa fever, Ebola, and Marburg disease and several other viral hemorrhagic fevers–and a few are not usually deadly but are unpleasant, like legionnaires disease and Lyme disease. Children are the main victims of many of these new diseases.

The development of agriculture ensured reliable food supplies, and this led to a population surge. Little clusters of families and clans grew into towns and cities that were the essential prerequisite for the rise of civilization–religions, laws, arts and culture, science, and literature. But increasing population density and a closer relationship of humans to domesticated animals such as pigs, sheep, cattle, and goats, transformed ecosystems, shaped the evolution of microorganisms, and increased the risk of transmission of pathogens.

In medieval times, cities and towns consisted mainly of ramshackle dwellings resembling the shanty towns surrounding modern cities in low-income countries. Domestic refuse, kitchen waste, and human and animal excreta were often scattered indiscriminately in places where children played. These conditions provided a favorable habitat for innumerable pests–flies, cockroaches, rats–which contribute to the transmission of several contagious diseases. Most people seldom bathed and changed their clothes infrequently, so they and their clothes were usually filthy and infested with vermin, a haven for lice and fleas–the vectors for two deadly diseases, typhus and plague. Pollution of water with human and animal excreta led to incessant outbreaks of contagious gastrointestinal diseases. Infectious diarrhea, spread by fecal contamination of drinking water or food, became even more prevalent as human settlements grew ever larger in the absence of adequate sanitary measures, reaching a peak at the time of the great cholera epidemics in the rapidly expanding industrial cities of nineteenth-century Europe and America. Pools of stagnant water as well as the rivers and lakes that attract human settlements provide breeding sites for mosquitoes, which are an intermediate host for malaria parasites. Malaria was endemic, with occasional epidemic flare-ups throughout much of Europe and America, especially in the Mississippi River valley, until early in the twentieth century. Different species of mosquitoes transmitted other deadly contagious diseases–yellow fever, dengue, viral hemorrhagic fevers, viral encephalitis. Children were the principal victims of all these diseases.

As human settlements expanded and spread throughout the world, people came together in ever greater numbers. Increasing population density, trade, commerce, wars, and conquest led to frequent contacts among strangers. Many who survived the initial attack of contagious diseases acquired immunity, which protected them on subsequent exposure. But rising population density did not necessarily help to enhance immunity. On the contrary, it facilitated epidemic and pandemic spread of diseases, notably of those associated with lice and fleas, typhus and plague. Commerce and European exploration from the fifteenth to the nineteenth centuries led to increasingly frequent contact among people whose habitats were far apart and whose resistance to contagious diseases, because of prior exposures that conferred some inherited immunity, were very different. The consequences for those with no prior exposure were often disastrous. European conquests and colonization of the Americas were assisted by lethal epidemics of measles, smallpox,

and tuberculosis, all of which had far higher death rates among the indigenous peoples of the Americas than among their European colonizers. Smallpox decimated the Aztecs, facilitating the Spanish conquest. Measles killed an estimated 90 percent of the people of Fiji in a single epidemic.

The Heightened Vulnerability of Children

Children have always been more vulnerable than adults to infection with most contagious diseases, for several reasons. Children are innately affectionate and undiscriminating in hugging and kissing playmates, pets, and others. Young children, and toddlers in particular, explore their environment in part by tasting it–this is an inherent behavioral characteristic of all small children. Inevitably these behaviors expose children to frequent and repeated invasion by infectious pathogens of every kind. Most children are born with a partial immunity to common infections acquired from maternal antibodies and reinforced by additional antibodies in breast milk. But this acquired immunity wanes when children are weaned, so by the time children are two to three years old, resistance to common contagious diseases is often low or nonexistent. This explains why young children have always been more prone to contagious diseases than adults, since adults are those who survive infections in childhood and acquire some immunity.

Malnutrition, which has always been common, impairs children's immune responses, making it more likely that infection can overwhelm their bodily defenses more rapidly than those of adults. Children's respiratory systems are smaller than adults'; their respiratory passages are narrower and more readily blocked by inflammatory swelling and exudates of mucus. All these factors contributed to the high risk of a fatal outcome when children were attacked by contagious diseases in the centuries before development of protective vaccines and modern treatment methods.

Varieties of Contagious Diseases

The contagious diseases that have afflicted children and infants throughout history can be classified according to their mode of spread and means and route of invading the human body.

1. Diseases spread by the fecal-oral route are transmitted by polluted water or contaminated food, milk, and so forth. They include several varieties of acute gastroenteritis that are characterized by diarrhea and vomiting. One of these diseases, cholera, has occurred in a series of worldwide pandemics; but over the centuries since medieval times, acute infantile gastroenteritis has had a higher toll of infant and early child deaths than cholera, because it was constantly present whereas cholera occurred only in occasional albeit devastating epidemics. Diarrheal diseases still kill 3 to 4 million infants and children every year, mainly in low-income countries in Africa and Asia. Other diseases spread by the fecal-oral route include poliomyelitis, which does not cause gastrointestinal symptoms but invades the nervous system after its entry into the body, and viral hepatitis, an inflammatory disease of the liver. Both polio and hepatitis were almost universal and mostly either symptomless or mild infections of infants and young children until improved sanitation in the twentieth century reduced the risk of acquiring them early in life; older children and adults often are vulnerable to severe attacks of these diseases, and this explains why polio and hepatitis became such prominent and dangerous contagious diseases of older children and young adults in and after the second or third decade of the twentieth century.
2. Diseases that invade or attack the respiratory system (throat, windpipe, lungs) are primarily spread by droplets, such as the spray or mist suspended in the air after an infected person coughs or sneezes. Diseases spread by droplet include tuberculosis, bronchitis and croup, diphtheria, measles, scarlet fever, and whooping cough. The most common dangerous forms of acute respiratory disease, acute bronchitis and bronchopneumonia, still kill about 3 million infants and children every year in low-income countries. Diphtheria attacks the wind-pipe, causing its little victims to choke to death; the diphtheria bacillus also produces a toxin that damages the heart. Measles causes acute inflammation of the lungs–bronchitis and broncho-pneumonia. It is often lethal when the affected children are malnourished. Until early in the twentieth century it was common for several children to share the same bed. Older children with measles or other contagious diseases often infected their younger siblings, with deadly consequences. Improved living conditions did more than anything else to reduce the role of measles, croup, and whooping cough as major child killers. By the time the measles vaccine was developed, death rates from measles had fallen to insignificant levels in the rich industrial nations. Tuberculosis is further discussed below.
3. Diseases transmitted to humans by insect vectors include several that have been terrible scourges of humanity–malaria, plague, typhus, and yellow fever. Nowadays these diseases mainly afflict people in low-income tropical countries, but until late in the nineteenth century all occurred in temperate regions of Europe and North America. Environmental and social conditions are important determinants of all of these vector-borne diseases, as discussed below.
4. Diseases spread by direct person-to-person contact are mainly sexually transmitted or mother-to-infant transmitted. They include several varieties of hepatitis and the current worldwide pandemic of HIV/AIDS, which so far has affected over 40 million people and killed more than 10 million. About two thirds of the affected live–and die–in sub-Saharan Africa. HIV/AIDS is having a catastrophic impact on child health in Africa, both directly, when it kills infants and children born to HIV-positive mothers, and perhaps even more so indirectly, when children orphaned after both parents have died of AIDS are left to fend for themselves.
5. Diseases caused by pathogens that reside in the environment include tetanus. Until anti-tetanus vaccine was developed in the 1930s and then widely used in vaccination campaigns after World War II, tetanus was among the great killing diseases of newborn infants. When tetanus spores lodge in the cut end of the umbilical cord, they produce a toxin that causes death within a couple of weeks of birth. Neonatal tetanus has always been associated with dirty midwifery and with unhygienic traditional childbirth practices. As recently as the 1960s it killed over half a million newborn infants every year. Even now, with vaccination of pregnant women to protect their unborn children, preserving antiseptic precautions during obstetric procedures is the best way to prevent infant deaths from neonatal tetanus.

The Great Killers

Throughout history, epidemic and pandemic contagious diseases have struck down prodigious numbers of people, sometimes damaging the fabric of society so gravely as to disrupt commerce and industry. Early in the twenty-first century this is happening in sub-Saharan Africa, where HIV/AIDS is devastating whole nations. The progress of European civilization was disrupted by the pandemic black death (almost certainly bubonic and pneumonic plague) that struck from 1347 through 1349. The nineteenth-century cholera epidemics, while less disruptive, had a comparable short-term impact, as have several epidemics of typhus and the worldwide influenza pandemic of 1918 through 1920, which caused an estimated 20 million deaths. Other diseases that have exerted a longer-term and ultimately more damaging effect on human affairs are smallpox, malaria, and tuberculosis. Several classes of contagious diseases, and the measures taken to control them, require further discussion.

Plague, caused by bacteria that are carried by fleas, is primarily a disease of rats, but when it kills the rats, their fleas seek other mammalian hosts, carrying the plague bacillus with them. Bubonic plague is so called because the infection causes large swellings, or buboes, in the armpits and groin. In humans, plague often infects the lungs, causing fatal pneumonia, or it invades the blood stream and causes generalized septicemia. It is among the most deadly of all pandemic diseases. The pandemic known as the black death killed about a third of the population of Europe between 1347 and 1349; it arrested the advance of European civilization for several generations. Most of the victims were children. Plague gave rise to the legend of the Pied Piper of Hamelin, and to the nursery rhyme "Ring-a-ring o' roses," both commemorating the terrible mortality among children. Epidemics of plague were associated with squalid living conditions that encouraged infestation with rats.

Epidemic typhus, like plague, relies on insects to assist its transmission. In this case it is the body louse, which flourishes in unwashed clothing on unwashed bodies. Typhus was typically a disease of campaigning armies, but it attacked townspeople too, so it was a child killer in epidemics from the Thirty Years War to the early twentieth century and it reappeared in the ghettoized populations of European Jews during World War II. Like the plague, its control depended mainly on eliminating the insect vectors–the lice that transmitted it. Cleanliness and frequent washing and changes of fresh clothing are the best way to prevent typhus. In 1944, an impending epidemic in Naples was prevented by widespread use of the insecticide DDT to kill the lice with which many of the people were infested.

Malaria remains extremely common in humid tropical regions where the mosquito vectors flourish. Formerly it was common in southern and central Europe, in England as far north as the Midlands, and in South, Central, and North America as far north as southern Canada. It killed innumerable children. It has been treated for centuries with quinine and more recently with modern quinine derivatives. But as with all the great killers, a population-based strategy of prevention is more effective than treatment of individual cases. The strategy relies on eradication of mosquito breeding sites, use of insecticides to kill mosquitoes, and window screens and netting surrounding beds to protect people from mosquito bites indoors and while they are asleep. This strategy was widely applied throughout many malarial regions of the world, including the southern United States from the early twentieth century onward, and has successfully eliminated malaria from most places except sub-Saharan Africa and parts of southern and Southeast Asia. At the beginning of the twenty-first century malaria kills about a million children annually, most of them in Africa.

Cholera has always been the archetype of fecal-oral infections. It existed in southern and Southeast Asia from ancient times. It was imported to Europe and North America when trade expanded in the eighteenth century and reached a peak in great lethal epidemics in the nineteenth century. Transmission was by polluted water, consequent on lack of sanitation and inadequate personal and kitchen hygiene. Control came with the introduction of sanitary disposal of human excreta and provision of safe water supplies. Cholera continues to flourish in southern and Southeast Asia. An epidemic occurred in the Pacific coastal regions of South America in 1990, caused by import of the cholera vibrio in ballast water on ships from the Bay of Bengal, combined with complex ecological factors associated with climate change, but the high death rates of former generations have been controlled by oral rehydration therapy.

Smallpox, a viral disease, has scourged humans since before the dawn of history. Great epidemics often cut a swath through the population, killing up to a third of all attacked, scarring many who survived and causing blindness when it affected the eyes. As with all epidemic contagious diseases, children were the main victims. In the late eighteenth century, the English family doctor and naturalist Edward Jenner experimented on twenty-three child patients. He inoculated them with cowpox lymph and reported that this protected them against smallpox in an epidemic then raging in England. Jenner's 1798 book, An inquiry into the causes and effectsof the variolae vaccinae may be the most important treatise on public health ever written; it stimulated vaccination programs against smallpox and led directly to the eradication of smallpox (which was officially proclaimed by the World Health Assembly in 1980). Samples of smallpox virus survive in secure laboratories in several countries and are a potential weapon in biological warfare.

Tuberculosis, also called the white plague or consumption, ravaged the people of Europe and North America in the eighteenth and nineteenth centuries. Typically it struck when its victims were young, often teenaged, but it usually did not kill them until they were in their third or fourth decade, or older. Its direct cause is the tubercle bacillus, but it hits hardest in the presence of poverty, poor nutrition, over-crowding, ignorance, unemployment, and alcohol abuse– a common combination among poor populations in the squalid industrial cities of nineteenth-century Europe and America.

Control of Contagious Diseases

Treatment of individual cases of contagious diseases was largely ineffectual until the discovery and development of antibiotics and other modern antibacterial remedies, which did not occur on a wide scale until midway through the twentieth century. Treatment of individual cases, however, remains a poor substitute for public health measures aimed at preventing contagious diseases from attacking the population. In only one group–sexually transmitted diseases–is treatment of individual cases an effective means of control.

Effective control requires an understanding of the epidemiological features of contagious diseases, which in turn requires an understanding of their ecology. Vaccination and immunization programs have successfully controlled many contagious diseases, including smallpox, measles, whooping cough, diphtheria, and tetanus, but sometimes control of contagious diseases has occurred as a byproduct of societal changes, and the epidemiological insights and preventive measures have come later, after the waning of the great epidemics.

Ecological Approaches to Control

Although the necessary cause of each of the contagious diseases is a specific pathogen, other determining factors are required before a lethal epidemic occurs. The phrase filth diseases captures the sense of values that developed in the late nineteenth century as understanding about contagious diseases increased. Public health scientists and administrators recognized that effective control required an attack on what came to be regarded as filthy living conditions and behavior. Dirty, vermin-infested housing; squalid, overcrowded living conditions; fleas, lice, flies, and rats; lack of sanitation; and poor personal and domestic hygiene were all recognized as important environmental, social, and behavioral determinants. Effective control required attention to social and economic conditions, education about improving personal and domestic hygiene, as well as vaccination and immunization against vaccine-preventable diseases, sanitary engineering, and the provision of safe water supplies.

The Future of Contagious Diseases

The insights of history enable us to make educated guesses about possible futures. About halfway through the twentieth century, the optimistic view prevailed that soon all contagious diseases would be controlled and eliminated by the use of vaccines and antibiotics. The optimism was short-lived. It soon became apparent that antibiotic-resistant strains of dangerous pathogens would present an ongoing problem. Then deadly new contagious diseases emerged. This process of emergence continues. It is an inevitable consequence of evolutionary biology–pathogens will always continue to evolve, and because their reproduction rates are so prodigious and generation times so short, microorganisms evolve very rapidly. This explains the rapid development of antibiotic-resistant strains of many pathogens. The role of public health sciences is to anticipate the process and have public health practices ready to cope with each new contagious disease as it emerges.

See also: Infant Mortality; Pediatrics .

bibliography

Chadwick, Edwin 1965 [1843]. Report on the Sanitary Condition of the Labouring Population of Great Britain. Reprint, annotated by M.W. Flinn. Edinburgh, UK: Edinburgh University Press.

Chin, James, ed. 2000. Control of Communicable Diseases Manual, 17th ed. Washington DC: American Public Health Association.

Ewald, Paul. 1994. Evolution of Infectious Disease. New York: Oxford University Press.

Kunitz, Steven J. 1994. Disease and Social Diversity: The European Impact on the Health of Non-Europeans. New York: Oxford University Press.

McMichael, Tony. 2001. Human Frontiers, Environments and Disease: Past Patterns, Uncertain Futures. New York: Cambridge University Press.

McNeill, William H. 1976. Plagues and Peoples. New York: Doubleday.

Porter, Roy. 1997. The Greatest Benefit to Mankind: A Medical History of Humanity from Antiquity to the Present. London: Harper Collins.

John M. Last