DAESH is a terrorist organisation with a Salafist jihadist ideology that threatens the civilized world. Moreover, like a contagious agent of a cancer cell to the rest of the body, the foreign terrorist fighters of DAESH are a growing threat against their states of origin, the states they transit, and the states where they are active, as well as their neighbouring zones. Turkey falls under all these categories. Between 2014 and 2016, 159 individuals in Turkey lost their lives because of terror attacks executed by the foreign terrorist fighters. Data from these attacks are analysed in the article. While increased international cooperation on the use of no-entry lists, has produced some positive improvements in the efforts against foreign terrorist fighters, such collaboration should extend to other measures as well. In conclusion, the article argues that while Turkey experienced the initial wave of terrorist attacks by returnees, foreign terrorist fighters might cause a metastasis to the rest of the world. Moreover, the contagious effect of foreign fighters for violence is not limited to DAESH and needs to be examined within other terrorist organizations.