Although youth generally is considered the healthiest phase of life, it is also a time of exploration and experimentation, with sexuality being a major area of development and change. Youth (aged 15‐24 years) comprise about 18% (1.2 billion) of the global population of around 6.7 billion people (UN 2007). The unique biopsychosocial changes associated with this age group results in disproportionate exposure and risk of HIV infection. Youth, therefore, are at the centre of the human immunodeficiency virus (HIV) pandemic (Ross 2006), but while estimates of the HIV burden of disease in adults (15‐49 years) are routinely made, it is more difficult to gauge trends in the HIV disease burden amongst youth.

One of the most vulnerable groups is homeless youth, many of whom are forced by circumstance to engage in a variety of risky behaviours and who often are least likely to seek prevention counselling, testing, and care. Unique interventions to reduce risks of HIV acquisition due to sexual activity and intravenous drug use in this vulnerable group therefore may be necessary.

The focus of this review is to evaluate and summarize the effectiveness of interventions for preventing HIV by modifying sexual risk behaviours in homeless youth.

Definition of terms
The terms "youth," "adolescent/teenager," and "young adult" often are used interchangeably but at the same time are context and culture specific. Since 1985 (International Youth year), the United Nations has referred to people between the ages of 13 and 19 as teenagers and between the ages of 20 and 24 years as young adults; youth are defined as being between 15 and 24 years of age. It is estimated that in 2004, half of all HIV infections occurred in youth, and about 98% of the 10 million youth living with HIV/AIDS worldwide were in middle‐ or low‐income countries (UNAIDS 2004).

The term “homeless youth” refers to a large variety of young people (i.e. runaway, throwaway, unaccompanied, street, and systems youth). Various countries have developed their own definitions of "homelessness" among youth which often are linked to the reason for their homelessness, with youth often emphasizing the throwaway and parents the runaway justification (Hammer 2002). A runaway is a youth who is away from home without the permission of parents or guardians; a throwaway is a young person who was asked to leave home by either parents/guardians or adults in the household (Hammer 2002). Girls seem more likely to be runaways (Hammer 2002) because of sexual abuse (Cauce 2000) while boys are more likely to be throwaways, possibly because of a greater likelihood of deviant behaviour (Moore 2006).

The term “street youth” can refer to homeless youth who live in high‐risk, non‐traditional places, such as under bridges, in abandoned buildings, and on the street, or to those whose lives are focused on the street (i.e., youth spending most of their time on the street and fending for themselves) but return to their homes on a regular basis. It should be noted that reference to the word "street" when discussing homeless youth may be considered derogative (Panter‐Brick 2002), and street youth can be defined further in terms of context, as follows:

o       Street‐living ‐ youth who ran away from their families and live alone on the streets

o       Street‐working youth who spend most of their time on the streets and fend for themselves but return home on a regular basis

o       Street‐family ‐ youth who live on the streets with their families.

Systems youth on the other hand are considered to be linked to government systems, such as juvenile justice or foster care, and are homeless due to abuse, neglect, or family incarceration or homelessness (Moore 2006). When a systems youth become homeless it is often because they have run away from a home placement or because the placement failed to provide a stable living environment (Moore 2006).

Against this background, it is clear that homeless youth are not a homogeneous group with similar contexts and needs. The hidden and transient nature of homeless youth make it difficult to estimate their numbers (Raleigh‐DuRoff 2004), primarily, among other reasons because service providers and concerned adults attempt to "protect" youth from police and social services (Kidd 2004; Taylor 2004). UNICEF (2004) estimates that there are about 100 million children who live on the street either all or part of the year. In the United States it is estimated that about 1.5 to 2 million youths per year are homeless or run away from home (Moore 2006). These estimates likely underestimate the number of youth who are homeless in a broader definition inclusive of orphans and vulnerable children (OVC) in contexts of poverty, war, famine, and HIV/AIDS. Homeless youth, therefore, are of particular importance as a population at high risk for acquiring HIV infection.

Why it is important that interventions in this population be reviewed?

In countries with focused and generalized epidemics, homeless youth are highly vulnerable to HIV acquisition and transmission, as they live lives of heightened personal and social risk often marked by violence, victimisation and human rights abuses (Boivin 2005; Greene 1997; MacLean 1999; Ramphele MA 1997; Ribeiro 2001). As a group, they lack the economic, social, and emotional resources to provide for their basic needs, such as adequate sleep, clothing, shelter, and nutrition, which has dire consequences for their physical and emotional health (Van Wormer 2003). Studies have indicated that homeless youth have significant health risks related to sexual behaviour and substance use and linked to increased risk for HIV/AIDS and other STIs (Anarfi 1997; Baybuga 2004; de Carvalho 2006; Richter 1995; Walters 1999). In a review of 52 studies pertaining to the health of street youth in industrialized countries, these youth had higher rates of hepatitis B, C, and HIV than their non‐street peers (Boivin 2005).

The behaviours of youth are influenced by a range of factors including individual knowledge and attitudes; relationships with parents, caregivers, and peers; academic/school performance; socioeconomic and environmental contexts; and access to health services. It is generally understood that the majority of youth are sexually active, do not have monogamous sexual relationships, and do not use condoms consistently, as is reflected in the increasing prevalence of HIV among youth worldwide (UNAIDS December 2007). Risk behaviours among youth are often understood in terms of risk clustering, as different risk behaviours occur simultaneously, such as substance use and unprotected sex and sometimes delinquency. These behaviours also seem to have common determinants, meaning that they are affected by the same risk and protective factors (UNAIDS 2004).

Knowledge of sexuality (Gatu 2000; Richter 1995) and HIV/AIDS among homeless youth is generally inadequate and not translated into preventive behaviours (Baybuga 2004; Bernier 1995; Gatu 2000; Robinson 2001; Snell 2002). Homeless youth are highly vulnerable to engaging in unprotected survival sex to meet their basic needs and/or feed a drug habit (Bernier 1995; Walters 1999) and are subject to rape (Swart‐Kruger 1997). Because of their psychosocial and contextual environments, they also are highly likely to have multiple sexual partners (de Carvalho 2006; Haley 2004; Poulin 2001; Richter 1995; Roy 2000; Swart‐Kruger 1997; van den Hoek 1997). Most are poorly equipped to combat sexual threats, as reported in an ongoing study of Montreal street youth, where only 13.2% of participants reported always using condoms during vaginal intercourse, and only 32.4% reported always using condoms during anal intercourse (Roy 2000). Homeless youth are more likely to engage in self‐destructive behaviors, including parasuicide, substance abuse (especially alcohol) and injecting drug use, than are other youth (Rew 2001; Ringwalt 1998; Roy 2004). Suicide and drug overdose were reported to be two primary observed causes of mortality among homeless youth in a Canadian study (Roy 2004). The engagement in self‐destructive behaviours is linked to psychological triggers, such as post‐traumatic stress disorder (Cauce 2000), low self‐esteem, and feelings of loneliness and hopelessness (Boivin 2005). These factors in turn accelerate and perpetuate the cycle of HIV vulnerability.

Because they are marginalized and underserved, homeless youth do not have adequate access to social and health services, including health education (Baybuga 2004; Snell 2002). Their HIV risk thus not only is related to their sexual practices, but also are linked to various predisposing and enabling factors that emerge from their psychosocial and contextual vulnerabilities (Lambert 2005).

In response to the need to address HIV/AIDS prevention worldwide, most countries have embarked on intensive mass media‐, community‐, and group‐level interventions and the introduction of widespread antiretroviral treatment programmes (UNAIDS December 2007) 2007). Randomised controlled trials undertaken in various countries have shown a reduction of high‐risk sexual practices by youth who have taken part in behavioural interventions (DiClemente 1995; Kegeles 1996; Ross 2006; Underhill 2008).

Interventions directed specifically at modifying the sexual risk behaviours of homeless youth have unique challenges. Implementing workable and practical programmes for heterogeneous mobile populations in non‐health care settings are difficult. Furthermore, for members of a group in which social and economic indigence may make it difficult to assert their sexual rights, HIV‐prevention messages could be viewed as irrelevant. Peer education and community‐based interventions have been implemented to reduce HIV risk among homeless youth. (Rotheram‐Borus 1993; Rotheram‐Borus 1997; Rotheram‐Borus 1999). It has been argued that the ability of the programme to respond to the needs of youth stems from creative service approaches in which youth are involved in program design and in delivering the programme near where youths gather (Tenner 1998). Participatory and tailored interventions seem to be effective in meeting the diverse needs of marginalised youth (Tenner 1998).

This review will identify and allow lessons learned from effective interventions to guide the development and implementation of future HIV‐risk reduction, behavioural modification interventions for homeless youth.