Urbanization has played an important role in the development of underdeveloped and developing countries. It is believed that the developed countries of the world could achieve fast economic progress due to large — scale urbanization. Urbanization during the industrial revolution helped fast development of some European Countries. However the environmental challenges have been quite difficult to deal with. As already highlighted, urbanization is a process in which overall development of a particular region takes place.

New industries are set up and this in turn helps to create large — scale employment which is much needed in developing countries. Due to employment, the per capita income of the people rises and the region achieves prosperity. Urbanization provides better infrastructural facilities, medical facilities and educational opportunities to millions of people. Urbanization also helps develop a scientific attitude among people. The overall standard of living of the people rises considerably because of urbanization. It  helps the region to prosper and cater to the needs of a large population. Urbanization provides plenty of opportunities for people from all sectors of society to excel and achieve their goals. Despite these advantages, urbanization is usually attached with numerous challenges among which includes:

Environmental pollution and Degradation: Pollution and degradation of the environment is one of the most prominent challenges. The increase number of vehicles on the roads and industrial wastes are the main source of air pollution.
Urbanization also leads to a rise in the noise pollution level. Contamination of sea, river and lake water due to various human activities is also a serious issue resulting out of fast urbanization. Land pollution is also a matter of great concern. These types of pollution can cause adverse effects on the health and well — being of people.
River pollution is particularly found to be worse where rivers pass through cites and the most widespread is contamination from human excreta, and sewage. In many cites in Nigeria for example, there is limited access to clean drinking water. The quality of several water sources is poor, with pollutant levels higher than the WHO’ standard (Alex; 2010). Sadly, pesticide contamination from urban agriculture, residues from sawmills and manufacturing industries, waste water from urban drains and municipal dumping of waste especially human excreta pollute drinking water sources that affect the health of the urban and peri-urban populations.
Population Explosion: this is a great rise in the population of a particular area over a time. It has been observed that urbanization and industrialization lead to an increase in the population of the region at a very fast pace. The population growth results in a situation where the facilities provided by the government are to be shared among many people. This can further cause inequitable distribution of wealth and resources. Providing the huge population with essential resources becomes a big challenge due to population explosion. It also puts a tremendous strain on the finances of the country.

Sanitation, water and transport problems:
The rise in the number of vehicle puts a huge strain on the infrastructure available for transport such as roads and railways. Water problems are quite common in many parts of urban cities. Keeping the cities clean and well maintained is surely one of the biggest challenges.

Housing: - providing shelter to a big population is in deed a big challenge posed by urbanization. The lack of space for construction of houses and building results in deforestation which causes several environmental problems. The problem of slums in urban cities is becoming more and more difficult to tackle due to the issue of displacement that the huge population residing on these slums will face.

Rising cost of living and wealth inequality: - The cost of living in urban is quite high compelling many people to live in poverty. Inequalities in the distribution of wealth which widens the gap between the rich and the poor is also a serious problem to deal with.

Increase in crime: - One of the biggest challenges is the increase in the crime rate. Survey conducted suggest by Charlie that the crime rate is much higher in urban cites as compared to rural settings. The rise in the number of crime is due to the need to earn wealth which results in people resorting to unfair means. The average person in the urban areas suffer the most because of the rise in criminal activities.

Health challenges: - urbanization is one of the major threats to health in the twenty first century. For the first time in history more than half of the world population resides in urban areas. Rapid urbanization is more pronounced in low to middle income countries of South East Asia and sub-Saharan African (WHO; 2010). About 34% of the total population of the South East Asia Region is urban. According to UN Habitat (WHO; 2010) over 40% of the urban population of South Asia live in slums. The urban poor suffer disproportionately from a wide range of diseases and health problems. Lack of safe drinking Water, poor sanitation and pollution from nearby transportation, factories and industrial complexes can harm the health of urban workers and their families. This is partly because of the degraded quality of basic environmental services such as clean air, water and soil. Poor urban dwellers often also lack access to health services and the cost of such services can itself be impoverishing. (WH0; 2OO7).
In Nigeria and other Africa countries, most cities are characterized by inadequate sanitation, poor waste management and poor state of public health and infrastructure which have led to the spread of wide variety of water — borne and other communicable diseases. Speaking to Business Day News Paper recently at the 2010 physician week organized by the Lagos state branch of Nigerian Medical Association (NMA) Edamisan Temiye, Lagos State Chairman of NMA, said that the growing urbanization has become one of the biggest health challenges of the 21st century to Nigeria and the African continent. According to Temiye health is a major urban policy issue in Nigeria because poverty and slum conditions pose a serious public health threat to the country’s rapidly expanding urban population. In his words “those of us who are living in the mega city of Lagos are very much aware of the constraints for space and the rapidly disappearing neigbouring meeting places for relaxation. We are well aware of the slums often located in the vicinity of comfortable living and well constructed neighbourhood. Our actions as individuals, community, corporate organizations and government should be focused towards taking actions to mitigate challenges of urbanization”.
Nigeria as a nation has over the years been experiencing an accelerated shift of her population from rural to urban areas. This rapid rate of urbanization has no doubt engendered several challenges and problems similar to situations in other parts of the world. Sadly most problems confronting cities, towns and their inhabitants include inadequate financial resources, lack of employment opportunities, spreading homelessness and expansion of squatter settlements, increased poverty, growing insecurity and rising crime rates, inadequate and deteriorating building stock, services and infrastructure. Other problems include sub standard and inadequate housing, slums, transportation problems, low productivity, crime, juvenile delinquency, lack of health and educational facilities, improper land use, insecure land tenure system rising traffic congestion, increasing pollution, lack of green spaces, inadequate water supply and sanitation, uncoordinated urban development and a,n increasing vulnerability to disaster. For Akin Osibogun, Chief Medical Director, Lagos University Teaching Hospital (LUTH) Idi-Araba, though urbanization has its positive effects, one of the health problems urbanization is confronted with is the issue of mental health as evident in the number of people with stress, depression and anxiety. He noted that urban cities have the increased risk factors for non- communicable diseases and their risk factors such as tobacco use, unhealthy diets, physical inactivity, harmful use of alcohol as well as risks associated with disease out breaks.
Lastly unplanned urban growth has also exacerbated communicable diseases including water-borne and food-borne diseases. Diseases like amoebiasis and tuberculosis thrive in overcrowded conditions and are exacerbated in dusty environments. Other health threats include diarrhea diseases, viral hepatitis, typhoid fever, HIV/AIDS, and vector -borne infectious diseases especially dengue fever.
The dynamic growth of cities can be an economic engine to lift developing countries out of poverty — but unplanned growth can lead to detrimental health consequences (WHO; 2010).

The close proximity of people is a prominent urban factor. Due to large number of inhabitants in cities, there is tendency of living close to each other.  The world’s densest cities are in Asia and with almost 30,000 inhabitants per KM2 which Murbai in India appears to be the highest.(Emilie et al; 2011).
Population density affects diseases, particularly those transmitted via respiratory and faecal oral routes like Entamoeba histolytica and Giardia intestinalis.    Because of high — population density there is an increase amount of shared air space, which leads to increased exposure to influenza, measles, and Mycobacterium tuberculosis.
 Urban centers usually have higher rates of tuberculosis infection than rural areas (Tienhardt; 2OO1,). For example in impoverished sub urban areas of Karachi, Pakistan, the prevalence of pulmonary tuberculosis is 329 per 100,000 people. much higher than the prevalence of 171 per 100,000 that was reported by Pakistan National Statistics (Akhtar et al; 2OO7) Densely populated cities also proved favourable grounds for the spread of emerging diseases, as shown by the severe acute respiratory syndrome or the recent, HINI   influenza epidemics. Careful urban planning is crucial to restrict household overcrowding, and provision of parks and open spaces relieves congestion.
In most low -income countries, urban expansion is due to natural population growth. However, migration of people from other urban centers or from rural areas contributes substantially to the growth of cities (WHO, 2009). Economic migration and forced displacement can contribute to population movements. In Amman, Jordan, at least 500,000 of the 2 million inhabitants have moved to the city seeking refuge from conflict or disaster area (UN- Habitat; 2010). For urban growth, migration is generally more important in nations with low rates of natural increase. In China for example, the floating population of rural migrants moving to coastal cities is about 120 million (Emile et al; 2O11). Large population movements are also occurring between cities, notably in Latin America and the Caribbean. In Sao Paulo, Brazil, a third of all urban growth can be attributed to migration from other cities (Harpham et al; 1995).
Migration therefore, affects the transmission of communicable diseases in several ways: first, cities might provide favourable conditions for the spread of germs that are imported by migrants. Schistosomiasis has established itself in urban areas most probably through infected migrants (Bella et al; 198O). The snail intermediate host of schistosoma Spp is present in urban water
bodies, and endemic foci occur in large cities such as in Bamako-Mali, Darel Salam-Tanzania and Kampala-Uganda (Dabo et al; 2003).
Second1y, migrants who are infected may also increase the transmission of diseases that is already present in a city. In Kinshasa, Democratic Republic of Congo, the massive inflow of internally displaced persons from provinces where African trypanosomiasis is endemic contributed to an increase of ten times in the number of cases report.
Finally rural migrants traveling from non-endemic areas might also change the epidemiology of infectious diseases already present in cities. Many new comers do not have the specific immunity for these diseases and are more susceptible to infections and more likely to develop severe forms than are residents (Huang et al; 2OO5)
Naive migrants also contribute to the spread of latent forms of the disease. In Kabul Afghanistan, where cutaneous leishmaniasis is endemic, most of the populations become infected and immune in early life. Since 1987, rural — to — urban population movements have substantially increased the risk of transmission among new comers, and the spread of this disease has reached epidemic proportion (Reyburn et al; 2OO3).

Migration of rural people can add to the urban burden of parasitic diseases in several ways. Firstly, if rural migrants travel from non endemic area they will be susceptible to parasite infections that are endemic in the destination city. For example rural migrants were reported to be at high risk of acute urban schistosomiasis in Cairo and Belo Horizonte, and of urban Cutaneous Leishmaniasis in afghan cities such as Kabul (Mott et al; 1990,).
Secondly, urban inhabitants may be at risk from new infections brought in by the rural immigrants. For vector-borne infections, the exact impact depends on whether (a), migrants themselves are infected by the pathogens in question. (b) The city is endemic for the disease (c) migrants carry the vector(s) or intermediate host(s) of the disease.
As the case may be, a non — infected rural migrant may bring the infected vector or intermediate host of an infection into a non — endemic city. This vector can then contaminate the urban environment, leading to infection among city inhabitants. Out break of chagas disease, filariasis, and schistosomiasis are all thought to have followed entry of their respective insect vectors or informed late hosts into metropolitan areas. It is also possible that new species of parasites could be introduced into urban areas if suitable vectors for their transmission are already present. New species of Schistosoma, Leishmania and filarial worms and new zyrnodemes of T. cruzi have been described in urban areas (Mott et al; 199O). However transmission of these parasites can only be sustained if the urban environment supports survival and multiplication of their vectors, facilitating continued spread between humans. A more complex scenario arises when an infected rural migrant or a rural migrant bringing an infected vector, moves into a non - endemic urban area. In either cases, transmission of a disease can be initiated or increased. This has led to the creation of hyper - endemic foci of Schistosomiasis in cities such as Darel salaam (Sudan), Harare (Zimbabwe) and Sao Paul (Brazil). It has also facilitated the transmission of chagas disease in the shanty towns of cities in Latin America and probably contribut to the appearance of malaria in several African, Asian and South American cities (Saker et al; 2004).

The population of people living in urban settings has in recent years increased explosively. In 1950, less than 30 percent of the World’s population lived in urban areas, but according to the World Bank estimates in 2003, population grew to 48 percent. If trends in migration and population growth continue, urban areas will witness a remarkable shift with about 60% of people living in urban settings in the world by 2030. The United Nations predicts that the world’s urban population will almost double from 3.3 billion in 2007 to 6.3 billion in 2050 and most of this increase will be in developing countries where health response systems are weakest (Emilie et al; 2011).
The urban populations of developing countries are expected to grow from 1.9 to 3.9 billion people, while the number of urban dwellers in developed countries will remain almost unchanged (Haumsou et al; 2010). For example in Africa, a number of mega cities have emerged (with more than ten million inhabitants) such cities are Lagos and Kano in Nigeria, Cairo in Egypt, Pretoria and Johannesburg in South Africa. In other mega cities of developing countries, like Mumbai, Mexico City and Sao Paulo, the populations are now approaching twenty million inhabitants each (Houmsou et al; 2010).
Urban population increase is a function of both rural emigration and expansion of existing city population, the relative importance of which varies by region (Saker et al; 2004).
In Latin America, metropolitan growth has largely reflected increased migration as a consequence of industrialization and the search for economic opportunities. In sub-Saharan Africa which
is currently the least urbanized region of the world, the region is undergoing the most rapid transition and urbanization which has resulted in rural impoverishment (Michael; 2000).
Several factors lead to increased population in cities of developing countries. These include. interstate war, internal conflict and political instability which has’ led to mass migration of people who found themselves as refuges and lived in ad-hoc interim accommodation in slums or peri-urban areas. Most of the world’s 6.9 million displaced people and refuges reside in refuge camps or temporary shelters in developing countries. In these often crowded environments, where provision of sanitation, clean water, food and health care services are typically inadequate, also where barriers to vectors and animals carrying infectious diseases are usually absent or insufficient, and where person to person contact is amplified, parasitic infections are common and often devastating. For example, after almost one million people fled from Rwanda to Zaire (i.e. present Dr. Congo) in l994,around 500 thousand died within a month due to epidemics of cholera and dysentery which broke out in refuge Camps (Centre for Disease control; 1995).
Conditions in refugee camps also favoured the out break of vector borne diseases, skin infections as well as intestinal helminthiasis. Malaria epidemics are well documented in refugee camps. For instance, in Afghanistan and Pakistan (Houmsou et al; 2O10) reported that the course of infection anthroponotic cutaneous leishmaniasis (ACL) in an Afghan refugee settlement in North Western Pakistan, in which over one — third of the inhabitants developed active lesions was thought to be, infected migrant carriers from Kabul.
Because urban populations are characterized by much higher densities of people, poor housing, inadequate sanitation, unsafe drinking water, more people sharing small spaces, diseases are therefore much more easily transmitted
Rapid and unplanned population growth places huge strains on a city’s infrastructure; often designed to protect the public health of a small urban elites this infrastructure is difficult to expand to meet the needs of a much larger population especially those living in shanty towns and slums at urban peripheries, where parasitic infections are shared among people.
In a metropolitan population of Nigeria, it was reported that school children whose parents live in slums and urban peripheries, were more prone to intestinal parasitic infection than their counterparts that live in residential areas and have affluent parents (Housmsou et al; 2OO9). These slums and peripheries were characterized by inadequate supply of clean drinking water, poor environmental hygiene with refuse dumps every where and tenuous health systems.
According to an estimation, about 50% of the world urban population lives at the level of “extreme deprivation” and about 70% in some cities (Stephen, 1995) At least one quarter of people living in urban areas in developing countries do not have access to safe water and 30% to 50% of solid wastes generated in developing countries urban centers are left uncollected (WHO/UNICEF; 2000). For example in the Eastern Korogocho slum area of Nairobi, Kenya more than 100,000 people live around a dumpsite of 13 hectare where children grow up deprived of basic services such as water and electricity and play on smelly waste ground, around rotting food, broken bottles, medical waste and much more. A 2007 UN Environmental programme (UNEP) report highlighted the prevalence of respiratory and gastrointestinal problems, skin infections among residents living close to the dumpsite (IRIN;2OO9).
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