In essence, this definition involves three elements: the transport of the person, the force or coercion of the victim, and the abuse and exploitation [13]. The United Nations Office on Drugs and Crime divides the definition of human trafficking into three sections: the act, means, and purpose [14]. The act, or what is done, generally refers to activities such as recruitment, transportation, transfer, harboring, or receipt of persons. The means of trafficking consists of threats or use of force, coercion, abduction, fraud, deception, abuse of power or vulnerability, or giving payments or benefits to a person in control of the victim. Finally, these acts are carried out for the purpose of exploitation, which includes prostitution, sexual exploitation, forced labor, slavery or forced servitude, and the removal of organs [14].

It is important to remember that human trafficking is not human smuggling. Human smuggling involves an individual being brought into a country through illegal means and is voluntary. The individual has provided some remuneration to another individual or party to accomplish this goal [7].

The TVPA of 2000 is a U.S. federal statute passed by Congress to address the issue of human trafficking and offers protection for human trafficking victims [15]. This statute defines sex trafficking as, "the recruitment, harboring, transportation, provision, or obtaining of a person for the purpose of a commercial sex act" [15]. A commercial sex act is, "any sex act on account of which anything of value is given to or received by any person" [15]. In other words, it usually involves the illegal transport of humans into another country to be exploited in a sexual manner for financial gain [27]. However, it does not always involve the transport of victims from one region to another; such cases are referred to as "internal trafficking" [28]. Victims of sex trafficking could be forced into prostitution, stripping, pornography, escort services, and other sexual services [29]. Victims may be adult women or men or children, although there is a higher prevalence of women and girls. The term "domestic minor sex trafficking" has become a popular term used to connote the buying, selling, and/or trading of children younger than 18 years of age for sexual services within the country, not internationally [29,30]. An element of force, fraud, or coercion is not necessary, as the victims are children and inherently vulnerable [30]. In the United States, the children most vulnerable to domestic minor sex trafficking are those who are homeless, abused, runaways, and/or in child protective services [29].

Although controversial, it is said that sex trafficking victims differ from consensual sex work in that sex trafficking victims are forced to involuntarily perform sexual services and are often not paid for their "work." Sex trafficking involves the use of force and coercion and can encompass other forms of criminal sexual activities, including forced erotic dancing, "mail-order brides," and pornography [28]. On the other hand, individuals involved in consensual sex work make a decision to provide sex services for a fee. The decision to enter sex work does not eliminate the possibility of being a victim of trafficking if one is held against his/her will through physical and/or psychological abuse [4]. It is also important to remember that this does not necessarily mean sex work is a choice these individuals would have made if other options were available or that they have a choice in selecting their sexual partners and/or sexual activities [31].

Domestic servitude refers to a category of domestic workers (usually female) who work in forced labor as servants, housekeepers, maids, and/or caregivers, often in private homes. In some cases, young women are lured with the promise of a good education and work, and when they arrive in the United States, they are exploited economically, physically, and/or sexually. Their passports or identification papers are taken away, and they are told they have to pay off the debt incurred for their travel, processing fees, and any other bogus expenses. Because they do not speak English, they find they have no other recourse but to endure exploitative working conditions [36]. Unfortunately, as in many sectors of forced labor, there are no regulations to monitor the conditions under which domestic servants operate [35].

It is estimated that at any one time up to 300,000 children younger than 18 years of age are serving as child soldiers [46,47]. Traffickers prefer to recruit children to serve as soldiers because they are inexpensive and more easily molded and shaped to comply and obey without question [48]. It can be difficult to comprehend the atrocities that these children witness and experience [49,50].

The rampant use of digital technology, such as the Internet, greatly facilitates sex trafficking. The relative anonymity of online contact can empower traffickers to recruit or sell victims. Graphic images of women and children engaged in sexual acts can be easily disseminated over the Internet [57]. Traffickers may employ the Internet for advertising, marketing to those interested in making pornography [57]. In addition, social media sites such as Facebook, Craigslist, and Instagram have been used as a means of facilitating trafficking (e.g., by connecting and grooming potential victims) [58,59,60]. Newsgroups offer opportunities for those interested in locating women and children for sexual exploitation.

Although many are careful in linking cultural factors to the etiology of human trafficking for fear of imposing judgment on a particular culture, many maintain that cultural ideologies that tolerate sexual trafficking, bonded labor, and child labor may be a stronger factor than poverty in predicting trafficking rates [36,42]. For example, some cultures emphasize collectivism and prioritizing the needs of the family and group first before the needs of the individual. Some children may feel they have to sacrifice themselves for their family when traffickers promise money [36]. Traffickers also know that they can threaten to hurt victims' families to keep them from escaping [36].

Furthermore, in many cultures, boys are more highly valued than girls, and as a result, girls are considered more dispensable [36]. Sons are considered the family's social security, staying with the family while daughters marry into other families. Therefore, girls may be more likely to be sold into slavery than boys.

Child labor is also inextricably tied to cultural factors. In India, for example, child labor is common because it is believed that children in the lower levels of the caste system (i.e., the "untouchables") should be socialized early to understand their position in society [42]. It has been observed that when traditional cultural and societal norms about women's roles were relaxed in some European countries and more women entered the labor force, child labor decreased [42]. Ultimately, it is difficult to unravel the effects of poverty and culture because the pressures of poverty can lead families to use tradition as a justification to sacrifice young men, women, and children [42].

Ultimately, the conversation about human trafficking is complex, and to attempt to isolate the causes is beyond challenging. Multiple factors have been suggested as possibly predicting human trafficking, including macroeconomic factors (e.g., gross domestic product per capita), unemployment rates, female inequality, cultural oppression, and lack of protection of women's rights [68,69]. In one study, ease of land access to the destination country appeared to be a powerful predictor in terms of the number of individuals trafficked [68].

It has been suggested human traffickers employ five general strategies to recruit and traffic victims [6,70,71,72]:

In studies of trafficked women, headaches, fatigue, dizziness, back pain, pelvic pain, stomach pain, sexually transmitted infections (STIs), unwanted pregnancies, and gynecologic infections were common, generally the result of continual physical, psychological, and sexual abuse [30,73]. Victims of labor trafficking also experience health issues related to the type of work, workplace conditions, malnutrition, and violence [74]. It is important to remember that some of these somatic complaints, such as headaches, fatigue, and gastrointestinal problems, may be underlying symptoms of anxiety, depression, and stress [73]. Some cultural groups might not use the terms "depression," "sad," or "anxious," but may use metaphors and somatic symptoms to describe their pain, all of which are embedded within cultural ideologies. The most common culture-based idioms of distress are somatic symptoms. Some groups tend not to psychologize emotional problems; instead, they experience psychological conflicts as bodily sensations (e.g., headaches, bodily aches, gastrointestinal problems, and dizziness).

In studies of trafficked women, headaches, fatigue, dizziness, back pain, pelvic pain, stomach pain, sexually transmitted infections (STIs), unwanted pregnancies, and gynecologic infections were common, generally the result of continual physical, psychological, and sexual abuse [30,73]. Victims of labor trafficking also experience health issues related to the type of work, workplace conditions, malnutrition, and violence [74]. It is important to remember that some of these somatic complaints, such as headaches, fatigue, and gastrointestinal problems, may be underlying symptoms of anxiety, depression, and stress [73]. Some cultural groups might not use the terms "depression," "sad," or "anxious," but may use metaphors and somatic symptoms to describe their pain, all of which are embedded within cultural ideologies. The most common culture-based idioms of distress are somatic symptoms. Some groups tend not to psychologize emotional problems; instead, they experience psychological conflicts as bodily sensations (e.g., headaches, bodily aches, gastrointestinal problems, and dizziness).

Under normal circumstances, young children are still developing physically; however, such adverse conditions can halt their development. The lungs of adolescent boys typically experience the most rapid growth around 13 to 17 years of age; working in conditions characterized by excessive toxic dust or unclean air makes them more vulnerable to developing silicosis and fibrosis [77]. In the United States, young children participating in agricultural work are at risk of the major traumas associated with farm work, such as injuries caused by tractors or falling from heights, in addition to those injuries associated with repetitive stress and exposure to toxins. Children have thinner layers of epidermis, which make them more vulnerable to the toxicity of pesticides, and this can ultimately increase their risks for certain cancers [77]. Children working in gold mines do intensive digging, lifting, and transporting and mix mercury with the crushed ore, often with their bare hands. Mercury toxicity can lead to neurologic symptoms such as loss of vision, tremors, and memory loss [78].

Victims of trafficking experience a host of psychological, mental health, and emotional distress. Depression, suicidal ideation, substance use, and anxiety are typically cited mental health problems [30]. Post-traumatic stress disorder (PTSD) is also common given the trauma many victims experience, including physical and/or sexual violence and abuse; victims forced into sex work experience continual, daily sexual assault [81]. In a study of 192 European women who were trafficked but who managed to escape, the overwhelming majority (95%) disclosed that they experienced physical and sexual violence during the time of their trafficked experience [73]. More than 90% reported sexual abuse, and 76% reported physical abuse.

Trafficked victims experience fear from the start of their capture through the transit phase and after they arrive at their destination. During the transit stage, many victims experience dangerous border crossings, risky types of transports, injury, beatings, and sexual assault [75]. Upon arrival to their destination, many trafficking victims have been socially isolated, held in confinement, and deprived of food [82]. All sense of security is stripped from them—their personal possessions, identity papers, passports, visas, and other documents are taken [75,82]. The continual fear for their personal safety and their families' safety and the perpetual threats of deportation ultimately breed a sense of loss of control and learned helplessness. It is not surprising that depression, anxiety, and PTSD are common symptoms experienced by trafficked victims.

In a study of 164 survivors of human trafficking who returned to Nepal, the authors examined the extent to which they experienced PTSD, depression, and anxiety [83]. All of the survivors experienced some level of these disorders, but the survivors who were trafficked for sex experienced higher levels of depression and PTSD compared to those who were not trafficked for sex. In a study with Moldovan survivors of human trafficking, researchers found that six months after their return, 54% had a diagnosable mental health issue. Specifically, 35.8% met the diagnostic criteria for PTSD, 12.5% met the criteria for major depression, and 5.8% were diagnosed with an anxiety disorder [84].

There is also some evidence that trafficked victims may experience complex PTSD, a type of PTSD that involves an acute change of the victims' sense of self, their relationship with others, and their relationship with God or a higher being [85]. These persons direct anger inwardly (toward themselves) as well as toward their perpetrators, which results in a loss of faith in themselves and the world [82,85,86]. Perhaps due to self-directed anger and shame, some will engage in risky sexual behaviors, self-harm, and substance abuse. Some victims also have difficulty managing and expressing how they are feeling, while others experience dissociation [82].

Substance abuse is also common among victims. In interviews, trafficked women discussed how traffickers forced them to use substances like drugs and/or alcohol so they could work longer hours, take on more clients, and/or perform sexual acts that they could not normally perform [75]. Other victims used substances as a means to cope with their situations. Trafficked individuals who are gender and/or sexual minorities report shame, confusion, and sexual identity issues if forced into heterosexual relationships [86].

Children forced into labor experience grueling hours and are frequently beaten by their captors. Underage victims of domestic sex trafficking fluctuate through a range of emotions, including despair, shame, guilt, hopelessness, anxiety, and fear [87]. Depending upon the level of trauma, some engage in self-destructive behaviors like self-mutilation or suicide attempts. For some, their ambivalence toward the perpetrators may be confusing. On the one hand, they want to escape the abuse, yet simultaneously, they may have a sort of traumatic bond with the perpetrators [87].

Children forced into conscription will also experience a host of psychological symptoms. In a study comparing former Nepalese child soldiers and children who were never conscripted, former child soldiers experienced higher levels of depression, anxiety, PTSD, psychological difficulties, and functional impairments [88]. In another study of former child soldiers from the Congo and Uganda, one-third met the criteria for PTSD [49]. The researchers found there was a relationship between greater levels of PTSD symptoms and higher levels of feelings of revenge and lower levels of openness to reconciliation [49]. In-depth narrative interviews of former child soldiers from northern Uganda found that the children spoke of the violence and atrocities they witnessed without any emotion, as if they had removed themselves from their experiences [89]. This speaks to how the victims have to numb themselves psychologically in order to cope. The researchers also found that the children who lost their mothers were more traumatized by this experience than by the violence they witnessed as soldiers.

Some have argued that the diagnostic criteria of PTSD may not be easily applied to those from different cultures. As a result, it is important to assess for other psychiatric disorders, such as depression. Japan, for example, never used the PTSD diagnosis prior to 1995, despite the fact that they have a large and intricate mental health system [90]. Ultimately, PTSD cannot be universally applied to every culture and for every humanitarian crisis; therefore, if a human trafficking victim does not necessarily fall within the Diagnostic and Statistical Manual of Mental Disorders criteria for PTSD, one cannot necessarily conclude that they have not experienced trauma or are not traumatized [90].

Healthcare providers are often the most likely to encounter a victim of human trafficking under circumstances that provide an opportunity to intervene, and victims may be encountered in most mental health and healthcare venues. One study estimated that 30% to 87.8% of victims accessed medical services at some point during their trafficking [93]. Survivors may seek care in hospital emergency rooms, at local mental health authorities, urgent care facilities, family planning clinics, or outpatient medical settings for a variety of issues, including sexually transmitted infections, pregnancy, depression (including suicidality), injuries resulting from assault, substance abuse-related issues, and PTSD [94]. Because medical and dental appointments may allow for more privacy than a victim's other encounters, they may represent a unique opportunity for healthcare providers to intervene.

Several barriers exist that prevent survivors from self-disclosing their experiences, including [98]:

Being trauma-informed is a strengths-based approach that is responsive to the impact of trauma on a person's life. It requires recognizing symptoms of trauma and designing all interactions with victims of human trafficking in such a way that minimizes the potential for re-traumatization. This involves creating a safe physical space in which to interact with survivors as well as assessing all levels of service and policy to create as many opportunities as possible for survivors to rebuild a sense of control. Most importantly, it promotes survivor empowerment and self-sufficiency. Survivors should also have access to services that promote autonomy and are comprehensive, victim-centered, and culturally appropriate. Additionally, trafficking survivors share that one of the most important steps to being trauma-informed is to be survivor-informed [100].

If answers to these questions indicate that an individual may be a victim of human trafficking, one should contact the National Human Trafficking Hotline at 1-888-373-7888. Under the child abuse laws, practitioners who are mandated reporters and who are suspicious that a minor is being abused should immediately report the abuse. For more information regarding specific states' reporting requirements, please visit https://www.childwelfare.gov/resources/states-territories-tribes/state-statutes.

In addition, the following interviewing recommendations were published by the World Health Organization to encourage service providers to continually and ethically promote human trafficking victims' safety during every phase of the interviewing process [102,110]:

Care and services provided to victims can be organized into three distinct categories: immediate and concrete services at the time of rescue; services related to recovery; and long-term services pertaining to reintegration [113]. When trafficking victims are rescued, a great deal of counseling services and practical, day-to-day assistance will be required. Housing, transportation, food, clothing, medical care, dental care, financial assistance, educational training, reunification (for those who wish to return to their homeland), and legal aid are some of the concrete services needed [24]. Practitioners should connect, coordinate, and case manage these services as much as possible. During this stage, it is also important to understand victims' needs, their strengths, and their risks and vulnerabilities [82].

The needs of human trafficking survivors are diverse, and healthcare professionals should be prepared to refer these individuals to a wide variety of services. In the initial period, acute injuries, mental health crises, and stabilization (e.g., housing, safety) are the greatest concerns. However, many victims experience chronic health and mental health issues related to their traumatization and will also require referral to services that will allow healing throughout their lifetimes.