An estimated 100 million people worldwide are homeless.

Homelessness is the condition and societal category of people who lack housing and food, usually because they cannot afford a regular, safe, and adequate shelter. The term "homelessness" may also include people whose primary nighttime residence is in a homeless shelter, in an institution that provides a temporary residence for individuals intended to be institutionalized, or in a public or private place not designed for use as a regular sleeping accommodation for human beings.

A small number of people choose to be homeless nomads, such as some Romani people (Gypsies) and members of some subcultures.

Refuges for the homeless
There are many places where a homeless person might seek refuge.
Outdoors: On the ground or in a sleeping bag, tent, or improvised shelter, such as a large cardboard box, in a park or vacant lot.
Hobo jungles/Shantytowns: Ad hoc campsites of improvised shelters and shacks, usually near rail yards, interstates and high transportation veins.
Derelict structures: abandoned or condemned houses or buildings, abandoned cars, and beached boats, generally referred to as squatting.
Vehicles: cars or trucks are used as a temporary or sometimes long-term living refuge, for example by those recently evicted from a home. Some people live in vans, sport utility vehicles, covered pick-up trucks, station wagons, or hatchbacks.
Public places: parks, bus or train stations, airports, public transportation vehicles (by continual riding where unlimited passes are available), hospital lobbies or waiting areas, college campuses, and 24-hour businesses such as coffee shops. Many public places use security guards or police to prevent people from loitering or sleeping at these locations for a variety of reasons, including image, safety, and comfort.

Homelessness or emergency shelters
City run homeless shelters such as emergency cold-weather shelters opened by churches or community agencies, which may consist of cots in a heated warehouse, or temporary Christmas Shelters.
Inexpensive Boarding houses called flophouses offer cheap, low-quality temporary lodging.
Residential hotels, where a bed as opposed to an entire room can be rented cheaply in a dorm-like environment.
Inexpensive motels also offer cheap, low-quality temporary lodging. However, some who can afford housing live in a motel by choice. For example, David and Jean Davidson spent 22 years at a UK Travelodge
24-hour Internet cafes are now used by over 5,000 Japanese "Net cafe refugees". An estimated 75% of Japan's 3,200 all-night internet cafes cater to regular overnight guests, who in some cases have become their main source of income.]
Friends or family: Temporarily sleeping in dwellings of friends or family members ("couch surfing"). Couch surfers may be harder to recognize than street homeless people

Health care for the homeless
Health care for the homeless is a major public health challenge.
Homeless people are more likely to suffer injuries and medical problems from their lifestyle on the street, which includes poor nutrition, substance abuse, exposure to the severe elements of weather, and a higher exposure to violence (robberies, beatings, and so on).
Yet at the same time, they have little access to public medical services or clinics in America, in many cases because they lack health insurance: "Each year, millions of people in the United States experience homelessness and are in desperate need of health care services. Most do not have health insurance of any sort, and none have cash to pay for medical care."

Homeless persons often find it difficult to document their date of birth or their address. Because homeless people usually have no place to store possessions, they often lose their belongings, including their identification and other documents, or find them destroyed by police or others. Without a photo ID, homeless persons cannot get a job or access many social services. They can be denied access to even the most basic assistance: clothing closets, food pantries, certain public benefits, and in some cases, emergency shelters.

Obtaining replacement identification is difficult. Without an address, birth certificates cannot be mailed. Fees may be cost-prohibitive for impoverished persons. And some states will not issue birth certificates unless the person has photo identification, creating a Catch-22.

This problem is far less acute in countries which provide free-at-use health care, such as the UK, where hospitals are open-access day and night, and make no charges for treatment. In the US, free-care clinics, especially for the homeless do exist in major cities, but they are usually over-burdened with patients.

The conditions affecting the homeless are somewhat specialized and have opened a new area of medicine tailored to this population. Skin conditions and diseases abound, because homeless people are exposed to extreme cold in the winter and they have little access to bathing facilities. Homeless people also have more severe dental problems than the general population. Specialized medical textbooks have been written to address this for providers.

There are many organizations providing free care to the homeless in countries which do not offer free medical treatment organised by the state, but the services are in great demand given the limited number of medical practitioners. For example, it might take months to get a minimal dental appointment in a free-care clinic. Communicable diseases are of great concern, especially tuberculosis, which spreads more easily in crowded homeless shelters in high density urban settings.

International Law and Homelessness
Since the publication of the Universal Declaration of Human Rights (Charter of the United Nations -- UN) in 1948, the public perception has been increasingly changing to a focus on the human right of housing, travel and migration as a part of individual self-determination rather than the human condition. The Declaration, an international law reinforcement of the Nuremberg Trial Judgements, upholds the rights of one nation to intervene in the affairs of another if said nation is abusing its citizens, and rose out of a 1939-1945 World War II Atlantic environment of extreme split between "haves" and "have nots." The modern study of homeless phenomena is most frequently seen in this historical context.

Many non-profit organizations such as Goodwill Industries maintain a mission to "provide skill development and work opportunities to people with barriers to employment", though most of these organisations are not primarily geared toward homeless individuals. Many cities also have street newspapers or magazines: publications designed to provide employment opportunity to homeless people or others in need by street sale.

While some homeless have paying jobs, some must seek other methods to make money. Begging or panhandling is one option, but is becoming increasingly illegal in many cities. Despite the stereotype, not all homeless people panhandle, and not all panhandlers are homeless. Another option is busking: performing tricks, playing music, drawing on the sidewalk, or offering some other form of entertainment in exchange for donations. In cities where blood donation (plasmapheresis) centers still exist, homeless people may generate income through frequent visits to these centers.

Homeless people have been known to commit crimes just to be sent to jail or prison for food and shelter. In police slang, this is called "three hots and a cot" referring to the three hot daily meals and a cot to sleep on given to prisoners. Similarly a homeless person may approach a hospital's emergency department and fake a physical or mental illness in order to receive food and shelter.

Main causes of homelessness
There is a rarely a simple explanation for a person's homelessness. Homelessness is usually a consequence of a combination of both structural and individual factors. In the past, explanations of homelessness tended to concentrate on its being an individual problem due to personal difficulties. Now there is a much wider recognition of how societal factors such as social policy and social exclusion along with structural issues such as, poverty, unemployment and housing shortages contribute to homelessness. It is important in working to eliminate homelessness that both individual and structural factors are taken into account in order to come up with an appropriate and concerted response that meets the needs of the individual whilst addressing long-term causative factors.

Individual Factors
On an individual level, homelessness is frequently the result of a crisis in someone's life. There is a range of events or crisis points that can trigger homelessness, identified by Fitzpatrick and Klinker (2000) as being:

Leaving the parental home after arguments
Marital or relationship breakdown
Widowhood
Leaving care
Leaving prison
A sharp deterioration of mental health
Increased drug or alcohol misuse
A financial crisis or mounting debts
Eviction

In addition there are many factors that put individuals at a greater risk of homelessness when a crisis occurs. Common background characteristics of people who become homeless have been identified as:

Physical or sexual abuse in childhood or adolescence
Family disputes of breakdown
A background of institutional care
Offending behaviour and/or experience of prison
Lack of social support network
Debts, especially rent and mortgage arrears
Causing nuisance to neighbours (anti-social behaviour)
Drug or alcohol misuse
School exclusion and lack of qualifications
Mental health problems
Poor physical health

Structural Factors
Structural factors, relating to how we organise our society and distribute wealth and power, are widely acknowledged as being a pivotal factor in homelessness. The failure of infrastructure to support those most vulnerable in our society has resulted in high levels of poverty, rising unemployment, social exclusion, the lack of affordable accommodation and increasingly negative effects of de-institutionalisation ultimately creating pathways into homelessness.

The major reasons and causes for homelessness as documented by many reports and studies include:

Lack of affordable housing
Substance abuse and lack of needed services
Mental illness and lack of needed services
Domestic violence
Poverty, caused by many factors
Prison release and re-entry into society
Lack of affordable healthcare
Natural disaster
The high cost of housing is a by-product of the general distribution of wealth and income. The rate of homelessness has also been impacted by the reduction of household size witnessed in the last half of the 20th century.

Individuals who are incapable of maintaining employment and managing their lives effectively due to prolonged and severe drug and/or alcohol abuse make up a substantial percentage of the U.S. homeless population. The link between substance abuse and homelessness is partially caused by the fact that the behavioral patterns associated with addiction can alienate an addicted individual's family and friends who could otherwise provide a safety net against homelessness during difficult economic times.

Increased wealth and income inequality have caused distortions in the housing market that push rent burdens higher, thereby decreasing the availability of affordable housing.

There is an initiative in the United States to help the homeless get re-integrated into society, and out of homeless shelters, called "Housing First". It was initiated by the federal government's Interagency Council on Homelessness. It asks cities to come up with a plan to end chronic homelessness. In this direction, there is the belief that if homeless people are given independent housing to start off with, with some proper social supports, then there would be no need for emergency homeless shelters, which it considers a good outcome. This is a very controversial position.

In Boston, Massachusetts, in September 2007, an outreach to the homeless was initiated in the Boston Common, after some arrests and shootings, and in anticipation of the cold winter ahead. This outreach targets homeless people who would normally spend their sleeping time on the Boston Common, and tries to get them into housing, trying to skip the step of an emergency shelter. Applications for Boston Housing Authority were being handed out and filled out and submitted. This is an attempt to enact by outreach the Housing First initiative, federally mandated. Boston's Mayor, Thomas Menino, was quoted as saying "The solution to homelessness is permanent housing". Still, this is a very controversial strategy, especially if the people are not able to sustain a house with proper community, health, substance counseling, and mental health supportive programs.

Violent crimes against the homeless
There have been many violent crimes committed against the homeless. A recent study in 2007 found that this number is increasing.

Voting Rights
Over half the states in the USA require a person to have an address in order to vote. In this fashion, many homeless people are denied the opportunity to vote. Similar situations exist in many countries in the world.

Tracking the homeless
In the USA, the federal government's HUD agency has required federally funded organizations to use a computer tracking system for the homeless and their statistics, called HMIS (Homeless Management Information System). There has been some opposition to this kind of tracking by privacy advocacy groups.
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