In Islamic theology, a ‘Shaytaan’ can live among human beings or Jinns, for these two beings alone have been bestowed a ‘free will’ to choose between obedience and disobedience of their Lord among all His creations. All those among the Jinns and the human beings who follow the mission of disobedience of Iblis, the first Shaytaan are all termed Shayateen by Allah.
Shaytan are also "whisperers" who speak into the hearts of men and women alike to push them into doing sinful deeds and stray off the righteous path. This, and the Nafs "self which is inclined to evil" are the sources where the desire to sin comes from, according to Islam. There is a supplication in the Qu'ran which is aimed to fight off these temptations.
Shaytan can be used to refer to anything that harms or opposes you, such as harmful bacteria or viruses.