It has taken only 12 years for the world population to grow from 5 billion to today’s 6 billion. There is strong evidence that the growth of the world population poses serious threats to human health, socioeconomic development and the environment. The world population is growing at an alarming rate, and overpopulation will also be discussed as a public health issue.
Rapid, and often unplanned, urban growth is the source for many of the hazards faced by cities. Crowding, increasing levels of air pollution, water pollution and over usage, inadequate sanitation services and inadequate solid waste collection are all associated with rapid growth of urban centers. Rapid construction of poor-quality housing is a feature of many urban centers. For overpopulated communities, inadequate shelter and overcrowding are major factors in the transmission of diseases with epidemic potential such as acute respiratory infections, meningitis, typhus, cholera, scabies, etc. Outbreaks of disease are more frequent and more severe when the population density is high. Other public structures such as health facilities not only represent a concentrated area of patients but also a concentrated area of germs. In an emergency, the number of hospital-associated infections will typically rise. Decreasing overcrowding by providing extra facilities and a proper organization of the sites or services in health-care facilities is a priority.
Crowding increases the contact with the air and surfaces that other people breathe and touch. Diseases transmitted through respiratory and fecal-oral routes are more frequent in situations involving crowding, for example tuberculosis, rheumatic heart disease (caused by group A beta-hemolytic Streptococcus species, and helminthic infections. The closer people become, the easier airborne illnesses are spread, e.g. 8.6 million Tuberculosis cases in 2012. In addition, overpopulation will only create more polluted water supplies, e.g. approximately 3.4 million people die each year because of contaminated water related disease.
Disease agents (pathogens) need to get around and they have co-opted methods possible to move from one person to another. In addition, they need a place to reside where the host or the environment is not in a constant struggle to eliminate them, a friendly place safe from environmental hazards (ultraviolet radiation, desiccation, chemical agents) and host body defenses. This is called the disease reservoir or reservoir of infection, the source of pathogens from which a susceptible host may be infected. For many human diseases, the reservoir of infection is simply other humans with an active infection (such as the common cold). Carriers of pathogens also act as an efficient reservoir; sometimes the carrier is healthy (e.g. Typhoid Mary from the early 1900s or the 30% of the population that currently carries Staphylococcus).
Commercial globalization, population movements and environmental changes are the main factors favoring the international spread of microorganisms. Transport and communication development constitutes also a remarkable factor in the worldwide dispersion of micro-organisms. International travel, tourism and commerce are increasing, and they constitute an efficient transport system for pathogens and vectors. The mass movement of large numbers of people creates new opportunities for the spread and establishment of common or novel infectious diseases. Ships, airplanes or other vehicles can disseminate vectors of microorganisms, such as birds and insects. When insects arrive in a new environment, they have to adapt to the new ecosystem and establish themselves and dissemination to adjacent areas may then follow. An example of this situation is the West Nile virus, which was first identified in 1990 in blood collected from a woman from Uganda. This virus is a common pathogen in tropical Africa, the Middle East, and Eurasia. The first outbreak in western countries occurred in New York in 1999 and affected 59 persons. West Nile virus is disseminated by mosquitoes belonging to the Culex genus and by birds.
Zoonotic transmission of infectious diseases is facilitated by encroachment of human habitation and the loss of niche environments. However, the pathogen must first establish itself in the new environment, and pathogens that need a vector or an intermediate host will have a restricted distribution, depending on vector or host adaptation to, or availability in, the new environment. Non-zoonotic means of transmission, such as person–person or faecal–oral transmission of infectious diseases, can be affected by living conditions, size or density of population, sexual habits, and population levels of immunization.
Migration is the permanent relocation of an individual from one country to another. The number of international migrants has increased rapidly over recent years, i.e. 244 million in 2015. (UN Department of Economic and Social Affairs). Migrants are a diverse group (e.g. economic migrants, students, refugees, asylum seekers) and therefore the relationship between health and migration is complex. On the one hand, there is a social selection involved in migration (known as the ‘healthy migrant’ effect), since migrants are often younger and healthier compared to both their population of origin and people from the host country of a similar ethnicity. Most evidence of the healthy migrant effect comes from North America, where researchers have found that migrants have a health advantage, which diminishes as individuals become more assimilated into the host society.
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