Nonhuman primates are widely used in biomedical researches because of their genetic, anatomical and physiological similarities to humans. As a zoonotic pathogen, the monkey B virus (Cercopithecine herpesvirus 1) does not cause serious symptoms when it infects its natural host, macaques. However, this pathogen can result in lethal encephalitis, encephalomyelitis and neurological sequelae when it infects humans. This article will briefly discuss the reasons why the monkey B virus causes such serious consequences in humans.