At first glance, a foreign domestic worker in Hong Kong, a Rohingya migrant toiling on a fishing boat, a sex worker walking the streets of Mumbai and a child laborer cutting bamboo in a plantation in the Philippines have nothing in common.

But all four could be slaves, trafficked and exploited by criminals and employers profiting from the world's fastest growing illicit industry that is estimated by the International Labour Organization (ILO) to be worth $150 billion a year.

In the 15 years since a global treaty to combat human trafficking was adopted, "modern slavery" has gradually taken over as a catch-all term to describe human trafficking, forced labor, debt bondage, forced marriage and other slave-like exploitation.