Human trafficking is a significant social and public health problem that affects virtually all societies, but often it goes unrecognised and unreported. Human trafficking is a crime that has increased alarmingly in recent years, due to difficult living conditions in poor countries and the increasingly restrictive immigration policies in industrialised countries, among other factors. This form of human rights violation related to crime is expanding across all continents and is driven in part by the growth of violent conflict that produces refugees, failed states and the diversification and growth of markets related to sexual exploitation. In the same way, the delocalisation of productive processes linked to expanding consumption increases the possibilities for economic exploitation. For many years, human trafficking was not considered a structural problem but rather a series of isolated incidents (UNODC, 2014). Today we know that this is not the case. In fact, while we write this commentary for the Ottisova special article published in this volume, international media alert us to the fact that at least 10 000 unaccompanied child refugees from Syria have disappeared after arriving in Europe, according to Europol, the European Union criminal intelligence agency. Many are feared to have fallen into the hands of organised trafficking syndicates to work in informal workshops and in the sex industry (The guardian, 2016).

The work of Ottisova et al. brings to us a part of this problem: the consequences for the health of trafficked people. It does so through a systematic review of 37 articles published on this topic from 2011 to 2015 that update the prior review (Oram et al. Reference Oram, Stöckl, Busza, Howard and Zimmerman2012). In a clear and concise presentation of the data, the authors show the elevated prevalences – and when possible their calculation –, the relative risk compared with controls of mental, physical and sexual health problems, as well as physical and sexual violence.

The important health consequences for victims documented by this review show how they should be treated from the perspective of individual care. Thus, the health sector has an important role to play. Health care staff, well trained in engagement with survivors, is a prerequisite for inquiry about such experiences but is also necessary in order to be able to offer appropriate treatment and support – some guides with recommendations for serving health providers in the attention to human trafficking survivors serve as examples (Zimmerman & Borland, Reference Zimmerman and Borland2009). However, human trafficking should also be treated as a public health problem, and in addition, as a social problem. This double focus (as both a public health and social problem) begs the consideration of population-based strategies that can be adopted to approach the social determinants that give rise to human trafficking (Perry & McEwing, Reference Perry and Mcewing2013).

This review reminds us of the difficulties involved in developing research in this field. The population is rarely accessible, and as such, obtaining representative samples or reliable information is many times compromised. In the same way, important differences can be found in case definitions and even in the measurement of effects. However, despite these limitations, the review of articles carried out shows us that it is possible to find consolidated evidence to show without a doubt that this form of human rights violation has a dramatic impact on one's state of health. The authors of this commentary wish to highlight the value of this review in terms of raising awareness of the problem among the scientific community, and with authorities and other key social actors.

The review also reveals that research within this field needs to go on, continuing to improve knowledge. For example, there is the need to develop valid instruments to use with trafficked populations. There is also the need for studies with trafficked men and children. Although the majorities are victims of sexual exploitation, other forms of exploitation are becoming more frequent. Forced labour – a large category that includes, for example, the manufacturing, cleaning, construction, restoration, domestic work and textile production – has increased incessantly in recent years. About 40% of the victims detected between 2010 and 2012 were engaged in forced labour. It is an urgent topic for development from the perspective of occupational health research (Zimmerman & Schenker, Reference Zimmerman and Schenker2014).

By definition, human trafficking victims are invisible. The beneficiaries of this illegal industry avoid the contact of victims with public institutions, and victims are inaccessible to scientific research, except when they participate in post-trafficking support services, when they end their relationship with the criminal organisation that exploited them. They are also invisible to public opinion and the media. Despite the fact that this forced movement of people is not accounted for nor represented as potential seekers of asylum, they do not even have the option of requesting asylum or as economic migrants; they are not registered. Although invisible, victims become, by force, undocumented immigrants. This connection of human trafficking with migratory processes reminds us that political measures that affect the human rights of immigrants, particularly those directed against the undocumented, can have an especially negative impact on human trafficking victims; for example, when they restrict access to national health systems. It is positive that some countries favour the access of those who denounce human trafficking situations to residence permits and work as regular migrants, but even in these cases there are many problems related to finding the necessary support from public institutions to document their situations to the justice system.

One of the ways to make visible this phenomenon is to support actors who might have direct contact with victims, so that they contribute to denouncing these situations. Health professionals are especially implicated as are non-governmental organisations, social services, law enforcement and the justice system. The collaboration of these different actors, following guidelines for the documentation and denunciation of cases with the institutional support from within the system and integrating capacity to provide protection, is necessary to effectively provide protection to victims. It is not necessary to place responsibility on non-governmental organisations; rather, they should be integrated effectively in a coordinated strategy with institutions that aim to eradicate human trafficking.

In this area of work, it is important to be very sensitive to the structural and cultural characteristics of each society, since effective policies in a given geographical area can be irrelevant in other places. In the same way, an approach based on attention to diversity should be adopted, since just as common and universal aspects are shared by victims – as well noted in the review addressed by this commentary – they also have specific needs based on factors such as age, gender and ethnic group (Haase, Reference Haase2014).