Trends in a population's vital statistics, or demographic trends, within nations and across national boundaries have a profound effect on the distribution of infectious disease worldwide. Gender, age, the movement of populations due to economic opportunity or to escape conflict, and the sheer density of population relative to the capacity of local ecosystems, civic infrastructure, and public health resources all influence the infectivity and virulence of infectious diseases. Close quartering of a population such as in refugee camps, prisons, or schools can also affect the outbreak and spread of infectious disease.

Mass migration of a population to a host nation that has limited resources and is unprepared for the burden of caring for so many vulnerable people, in combination with close quarters and unsanitary conditions, often results in environments that are ripe for the transmission of pathogens. Movement of populations such as migrants or refugees affects the population itself, the populations encountered, and the ecosystem. Each translocated person carries cultural practices, genetic vulnerabilities and resistances to infections, and organisms that have been held at bay by the individual's immunity but lie dormant and are potentially dangerous to previously unexposed persons. In addition, the moving populations unwittingly transport microbes, animals that are disease vectors (transmitters), and other flora and fauna (plants and animals) that are foreign to the destination ecosystems.

One example of demographic factors coming together at once and contributing to disease was the mass exodus of more than a million Kurds from Iraqi villages at the end of the first Gulf War in 1991. With the absence of sanitary facilities and the crowding together of so many people in a weakened condition, contaminated water supplies from human waste quickly gave rise to an epidemic of cholera, as well as other communicable diseases. Beginning in 2003 conflict in the Darfur region of Sudan resulted in the movement of more than 1 million internally displaced people to crowded refugee camps where epidemics of typhoid, hepatitis E, cholera, and meningitis took hold.

Yet another example is that of the flight of 800,000 Rohingya people from Myanmar to Bangladesh. In 2017 there was a Burmese military crackdown on Rohingya insurgents that put many ordinary people in grave danger, causing them to flee across the border. So many Rohingya people (mostly women and children) arrived in Bangladesh that it became the center of the world's fastest-growing refugee crisis. The overcrowded conditions in the Bangladeshi refugee camps resulted in a resurgence of diphtheria, which had been nearly eradicated in the country. In the last two months of 2017, 2,526 suspected cases of diphtheria were reported, with 27 resulting in death. The Rohingya refugees were weakened by malnutrition, poor routine immunization coverage, and limited access to clean water and sanitary toilets, which can increase the risk for diphtheria and other infections.

Even when no mass population movements are taking place, the changing age and sex mix of stable populations over time can affect the spread of infectious disease. In other words, the population of potential disease hosts changes rather than remains stable. These changes affect the patterns of communicable diseases, particularly diseases that are sexually transmitted and that give rise to symptoms in one gender or the other (such as cervical cancer caused by human papillomavirus) or that attack people differentially in different age brackets, such as seasonal influenza, which usually infects older people more often than younger people.

Demographics such as the population density of various age, sex, and ethnic subgroups, along with other statistics that affect patterns of disease, can be made into mathematical models that help scientists map and predict Page 262  |  Top of Articleinfectious disease trends. These models involve various assumptions based on whether people can recover from infections, the rate of disease-related deaths, the development of immunity, and the duration of immunity (whether it is temporary or permanent). These models can also predict infectious disease catastrophes by location. For example, during the 1990s models showed that the persistence of the acquired immunodeficiency syndrome (AIDS) epidemic in many rural African communities reduced the population size to levels below those necessary to maintain the local population of the community. The models showed that AIDS was eliminating adults of reproducing age at a rapid rate. Increases in global AIDS treatment and prevention funding have ameliorated this problem to some extent in the African regions hardest hit by the AIDS epidemic, according to Avert's most recent report on eastern and southern Africa.

The simplest models often assume that the total population size is constant. For short-term outbreaks of a disease, simple disease models used to predict the course of an epidemic assume that the population is fixed and closed and depend only on the disease incidence and prevalence rates, disease duration (persistence), disease death rates, and occurrence of immunity. Models for an endemic disease (one that is naturally occurring in a region such as tuberculosis or malaria) usually assume that births and deaths balance each other so the population size remains unchanged. However, when the disease causes a significant number of deaths, as in the case of AIDS, this assumption is not realistic, and more complicated models assuming variable population are needed to predict the course of the epidemic. These sophisticated models incorporate assumptions about both birth and death rates, which can be influenced by the incidence and prevalence of disease as well as other factors. By the same token, population size influences the rapidity with which a disease is spread, with large, dense populations promoting the rapid spread of disease, and small, dispersed populations inhibiting such spread.

In the early 21st century, war, civil strife, and the breakdown of governance have become the predominant Page 263  |  Top of Articlemotivators of mass migration. The global scope of this development has resulted in the largest population of displaced peoples in history. During this period many populations have migrated because of war, civil unrest, ethnic cleansing, and genocide, in addition to reasons related to the economy and natural disasters. As the 20th century ended, about 150 million people (2.5 percent of the global population) were living outside of their country of origin, 10 percent of whom were refugees.

In 2018 nearly 1 in 100 people worldwide have been displaced from their homes, which is, according to the Pew Charitable Trust, “the world's population that has been forcibly displaced since the United Nations High Commissioner for Refugees began collecting data on displaced persons in 1951.” The regions with the highest percentages of displaced people are the Middle East (5.6 percent); continental Africa, excluding Egypt, which is considered part of the Middle East (1.6 percent), and Europe (0.7 percent). Population movements resulting from wars or violent conflicts have resulted in epidemiological outbreaks (e.g., cholera and typhoid fever), which arise from population overcrowding, malnutrition, unhygienic conditions, and lack of basic medical services.

The rise of bacterial resistance is another factor that has fundamentally changed the epidemiology of infectious disease. While community-acquired methicillin-resistant Staphylococcus aureus (MRSA) is most common in the United States, hospital-acquired MRSA has become the most common pathogen globally. The rates of antibiotic resistance are correlated with use (and overuse or misuse) of common antibiotics.

Seasonality is an important factor in the spread of common infectious diseases that most affect the youngest and oldest demographic groups (schoolchildren and the elderly). Illnesses such as influenza, measles, chickenpox, and pertussis (whooping cough) are all more prevalent at certain times of the year. Seasonality is a particularly important factor in models that predict whether these recurrent infectious diseases will occur in a given year or skip a year. Seasonal changes in disease transmission patterns and the susceptibility of a population to a disease (such as attending school or staying inside in close quarters during the winter) can prevent late-peaking diseases (disease epidemics that take a long time to reach peak infectivity) from spreading widely. When this happens, the remaining population is more susceptible to future epidemics because of a lack of herd immunity.

By analyzing seasonality and how much of the population remains susceptible to a disease, scientists can predict the course of newly emerging and reemerging diseases, such as West Nile virus, that are brought on by seasonal vectors (transmitters) including mosquitoes and migratory birds.

Of course, populations are not distributed uniformly even when they are stable and no significant migration is occurring. Infectious diseases spread in different patterns within a population that is divided into families or other groups than in a population that consists mostly of people who are living alone. A household constitutes a small population cluster, which is in turn composed of members that are resistant to the disease, along with members who are susceptible to the disease. An infectious disease spreads quickly and efficiently within the household, but the outbreak lasts longer if it spreads cluster by cluster or from one household to another.

The proportion of children and the elderly in a population is important in the spread of communicable diseases, particularly because both age groups are more likely than the general population to be in close quarters for extended periods in schools and in hospitals or nursing homes. In children, immune functioning is still developing, and they are constantly being exposed to pathogens that are familiar to adults but new to them. At the other end of the demographic scale, aging is associated with increased incidence and severity of many infectious diseases, including nosocomial infections. This increased risk is due to an age-related decline in the body's immune system function. As the average age of the population increases in industrialized nations, the epidemiology, morbidity, mortality, and needs for preventive action against nosocomial infections in the elderly also increase.

When an epidemic of a highly infectious disease is spreading in a community of households, the infection of any member of a household generally results in the infection of all susceptible members of that household. The rapidity of disease spread will thus depend on the household size and the variability of the number of susceptible people per household. If the rate of spread of infection from individual to individual within each household and the spread of infection from household to household are calculated, the rate and pattern of spread of the disease can be put into a mathematical model by public health scientists. This model can be used to calculate the levels of immunity that will be needed to prevent major epidemics in the community. It can also be used to evaluate alternative vaccination strategies that could immunize the same number of individuals.

For a community with households of approximately equal size (as seen in many suburban communities in the United States), random vaccination of individuals is better than immunizing all members of a fraction of households that would amount to the same total number of vaccinated people. On the other hand, when households vary widely in size (as seen in many US urban areas), vaccinating all members of large households can slow down the spread of the epidemic more rapidly than would the vaccination of an equal number of randomly selected individuals. This is Page 264  |  Top of Articlebecause disease transmission within these large households is easier than in the general community. Such epidemic spread models can also be used for a community of households with schools or day care centers. Immunizing every child within the school or day care center will be more effective than randomly immunizing an equal number of children in the community because the schools and day care centers are similar to very large households in which disease spread among many susceptible children is made easy by their close quarters.

Demographic characteristics of populations strongly determine the rate and extent of infectious disease distribution and spread. These demographic characteristics are in turn profoundly influenced by the processes of economic development, globalization, migration, and war. Although population demographics and patterns of infectious disease are in continual flux, they are rarely susceptible to policy-motivated human intervention. Rather, they are all aspects of the evolution of human cultures, which are intimately interconnected with evolving technology and commerce. The tools of epidemiological models that use demographic factors to help forecast the spread of infectious disease will constantly need to be updated as population characteristics change with increasing velocity in the years and decades ahead.

Recommendations for Counteracting Disease Spread through Migration

SOURCE: Blackburn, Christine Crudo, and Paul E. Lenze Jr. “Forced Migration and the Spread of Infectious Disease: Impact of Syrian Refugee Movements on Disease Prevalence in the European Union.” Scowcroft Institute of International Affairs. Texas A M University, November 2017. http://bush.tamu.edu/scowcroft/papers/blackburn-lenze/Paper%20No.%2010%20Blackburn%20&%20Lenze.pdf (accessed February 5, 2018).

INTRODUCTION: Large-scale migrations due to war and political unrest can have significant health impacts on both migrants and the communities that received them. In the following excerpt from a report of the Scowcroft Institute of International Affairs, researchers make recommendations about how best to reduce the impact of infectious diseases following the migration of Syrian refugees to Europe beginning in 2011.

The mass migration of people from Syria has put significant strain on the European Union. There has been increased economic pressure from supporting thousands of new arrivals, challenges with housing and community integration, and lastly new health issues that stress the public health infrastructure and put the health of citizens and refugees at risk. Many Syrians have traveled long distances, are malnourished, and may not have had appropriate vaccinations or access to any form of health care for long periods of time. We provide three recommendations for addressing the health issues posed by refugees coming into the EU from Syria that we believe can help mitigate the introduction of diseases.

The first recommendation is to provide training of local health care practitioners. Most of the diseases that Syrian refugees are bringing into the EU are not common to Europe, but they are common in Syria. The most common disease coming into the EU from Syria is cutaneous leishmaniasis, but it is not the only one. Educating health care professionals in Europe about the signs, symptoms, and method of transmission for the most common diseases appearing with the movement of Syrian refugees would help clinics to be better prepared to diagnose and treat the diseases when they identify their symptoms. Knowledge about the diseases would help eliminate delay in diagnosis and treatment and eliminating this delay could prevent a large-scale outbreak. If health care professionals are given all the tools they need to fight the new diseases, the threat to the European public and the refugee communities will be greatly reduced.

Our second recommendation is to provide health screening upon entry for those refugees entering the EU through formal channels. These screenings should include a routine medical examination, appropriate vaccinations, and testing for infectious diseases common in Syria. If infectious diseases are identified, the refugee should be started on the proper treatment and contained at the port of entry until the treatment protocol is complete. Once the treatment is complete they will be allowed to be integrated into the community. The purpose of the entry health screenings is to identify and treat diseases before they have an opportunity to spread into the population. European Parliament, the European Council, and the European Commission have all recognised that the health of refugees can no longer be ignored. Both Parliament and the Commission have committed millions of Euros to supporting the healthcare of migrants and have discussed the importance of identifying diseases and other conditions as they enter the EU.

Lastly, because large amounts of Syrian refugees are not entering the EU through formal channels, health outreach must be conducted in refugee communities, regardless of their legal status. Funding should be secured for health care teams to go out into refugee communities on a monthly basis offering free medical care, vaccinations, and infectious disease diagnostic tests. This will help the EU identify diseases that may be circulating in refugee communities and prevent them from finding their way into the Page 265  |  Top of Articlelarger population. Additionally, it provides refugees who may be afraid to seek health care because of their illegal status, the opportunity to be treated. Health care teams could also offer education and training regarding some of the most common diseases in the communities.