Police and aid groups say more than 60 percent of illegal prostitutes in Belgium are trafficked from Nigeria. Many are only teenagers and almost all come from Benin City, a city in the south of Nigeria.
Rosa was sexually exploited by Nigerian traffickers and had to prostitute herself on the streets of Spain, Norway, France and Belgium. But Belgian police saved her after two years.
"The police took me and asked me question if I want to talk. If I talk they are going to make a better way for me. They will give me document, I say yes because the stress is too much," she said.
Rosa - not her real name - was struggling in Nigeria, making ends meet for herself and her daughter. She was told she could marry a man in Europe. After crossing Morocco and reaching Spain by boat, she was told to repay a $55,000 debt and forced into prostitution.
Europol said last year that Nigerian human trafficking rings are one of the biggest challenges for European police forces.
Police now see those who were trafficked as victims, whether they have documents or not.
After speaking to the Belgium police, Rosa ended up in Payoke, a shelter for victims of sexual exploitation. There are three similar shelters in Belgium. Payoke has helped at least 4,000 women and witnessed a rise in Nigerian women from the early 1990s.
Payoke founder Patsy Sorenson says the shelter only helps victims who agree to file charges against the traffickers.
“The reason also that we ask their cooperation, is that we like to fight also against the traffickers," she explained. " It is a win-win situation also for them. When they cooperate we are able to offer them a lot of things. So that they are able to start a new life.”
A court case usually takes about two to three years. In that time, the shelter helps the girl get her life organized and after five years the victims can apply for Belgian citizenship.
Police commissioner Franz Vandelook says another big challenge is that most Nigerian illegal prostitutes end up trafficking and exploiting other girls once they have paid of their debt, meaning they will no longer be seen as a victim.
“They know very well what they have suffered in the past, and of course at a certain moment they decide to transform themselves to a madam too, because of the money of course," he said. "And they need money to feed the family who is still in Nigeria. So I can understand the situation, but in our society, in our European society, we can not accept the situation.”
Sorensen of Payoke says the women still face many challenges once they have decided to start a new life. Their family in Nigeria still needs money, their health is often a concern, many are still scared of the traffickers, and they often feel lonely while dealing with their traumatic experiences.
Rosa says when she started the court procedures, friends of the traffickers in Nigeria would beat her mother so badly she needed hospital treatment.
Despite the challenges, Rosa feels it was worth it.
“I can say now I am very happy because I am getting a good life now. Because before I was having a lot of stress, but now my stress is gone down. I can really say that I am very OK," she said.
The International Organization for Migration says last year about 37,000 Nigerians arrived by boat in Europe, about one-third of them women. It is estimated more than 8,000 of them will end up in prostitution.