Transmission by direct contact normally involves a bite by the infected reservoir animal or handling of its tissues or materials contaminated by its body fluids. Viruses usually replicate in the reservoir host tissues, and transmission occurs via fomites or direct contact between hosts or between the reservoir host and humans. Examples are the rodent-borne hemorrhagic fevers. The majority of viral zoonoses require a blood-sucking arthropod for transmission between hosts or to humans.
Mosquitoes are the most important vectors, followed by ticks, sandflies, and midges.
The viruses generally remain undetected until humans encroach on the natural enzootic focus, or the virus escapes the primary cycle via a secondary vector or vertebrate host. Humans and domestic animals generally become involved only after the virus is brought into the peridomestic environment by a bridge vector and are generally considered dead-end hosts because although they may become ill, they do not develop significant viremia to infect feeding vectors and thus do not contribute to the transmission cycle. The arthropod vector usually becomes infected when it ingests virus while feeding on the blood of a viremic animal. Virus replicates in the arthropod tissues, ultimately infecting the salivary glands. The arthropod transmits the virus to a new host when it injects infective salivary fluid while taking a subsequent blood meal.