Terrorism is a social disease both of individual societies and humanity as a whole and, as any illness, is the result of deviations from the natural course of their life. If contamination of human habitation gives rise to physical illnesses, and epidemics, "filth" of social habitation, i.e. social injustice, anarchy and violence of certain groups over others gives rise to dangerous social diseases, one of which terrorism is. To prevent the disease and treat them effectively after their appearance, it is necessary to define the correct diagnosis and to determine the causes of the disease. But so far, the attitude of people towards terrorism comes down to fight only its external manifestations without giving proper attention to their causes. In the article the problem of terrorism is considered with an emphasis on its main causes. Actuality of the studied problem is that terrorism as a social disease accepts in contemporaneous world the pandemic character and the reasons for this must be found. The article purports to establish the diagnosis of this disease to facilitate her treatment. Attentive and impartial analysis of the terrorist acts of the past decades by scholars from different countries gives the right to suggest that many of these acts were some kind of spontaneous responses to illegitimate acts of the states. Injustices of social relations, incapability of the institutions of power to effectively carry out their constitutional duties (protection of human rights and ensure their security) provoke some individuals and groups of people to no less unlawful and cruel acts. Terrorist movements are often the creations of the competing in different regions of the world States seeking "pull the chestnuts out of the fire by somebody’s ells hands." The article shows how some of the greater powers of the world, shortsightedly treating terrorists as additional tools in their fight against rival and providing them with material resources, generate their own enemies. This finding was confirmed in all studies on this issue, especially in studies of the English School of international relations. But so far, more attention has been given to show the activities of terrorist organizations, its effects and to justification of the need to combat them. This article investigates terrorism as a social phenomenon with an aim to establish its nature and causes, as well as effective means of its prevention. Materials and the conclusions of the study may be useful to the state institutions in defining their strategy for the protection of human rights, democracy and the rule of law, as well as for further research on the causes and ways of preventing negative phenomena in the life of societies and in relations between them.