Multiple strains of one virus can be present in the body at one time, and some of these strains may contain mutations that cause antiviral resistance. This effect, called the quasispecies model, results in immense variation in any given sample of virus, and gives the opportunity for natural selection to favor viral strains with the highest fitness every time the virus is spread to a new host. Also, recombination, the joining of two different viral variants, and reassortment, the swapping of viral gene segments among viruses in the same cell, play a role in resistance, especially in influenza.