Every lethal viral disease presents a paradox: killing its host is obviously of no benefit to the virus, so how and why did it evolve to do so? Today it is believed that most viruses are relatively benign in their natural hosts; some viral infection might even be beneficial to the host. The lethal viral diseases are believed to have resulted from an "accidental" jump of the virus from a species in which it is benign to a new one that is not accustomed to it (see zoonosis). For example, viruses that cause serious influenza in humans probably have pigs or birds as their natural host, and HIV is thought to derive from the benign non-human primate virus SIV.