For blood-borne viruses, transmission is more complicated. Generally it depends on a third party, a vector. The virus must replicate abundantly in the blood of the host to produce severe viremia (that is, a flood of virions). The vector (a blood-sucking insect or some other arthropod) must arrive for a meal, bite the host, slurp up virions along with the blood, and carry them away. The vector itself must be a hospitable host, so that the virus replicates further within it, producing many more virions that make their way back to the mouth area and stand ready for release.
