Transmission Many female mosquitoes, like those of Aedes albopictus, require a vertebrate blood meal in order for their eggs to develop.<unk> es maintain themselves in nature by going through a cycle between a host, an organism that carries the virus, and a vector, an organism that carries and transmits the virus to other organisms. For<unk> es, vectors are commonly mosquitoes, ticks, sandflies and other arthropods that consume the blood of vertebrates for nutritious or developmental purposes. Vertebrates which have their blood consumed act as the hosts, with each vector generally having an affinity for the blood of specific species, making those species the hosts. Transmission between the vector and the host occurs when the vector feeds on the blood of the vertebrate, wherein the virus that has established an infection in the salivary glands of the vector comes into contact with the host's blood. While the virus is inside the host, it undergoes a process called amplification, where the virus replicates at sufficient levels to induce viremia, a condition in which there are large numbers of viruses present in the blood. The abundance of viruses in the host's blood allows the host to transmit the virus to other organisms if its blood is consumed by them. When uninfected vectors become infected from feeding, they are then capable of transmitting the virus to uninfected hosts, resuming amplification of virus populations. If viremia is not achieved in a vertebrate, the species can be called a "dead-end host", as the virus cannot be transmitted back to the vector. A flowchart showing the West Nile virus transmission cycle. An example of this vector-host relationship can be observed in the transmission of the West Nile virus. Female mosquitoes of the genus Culex prefer to consume the blood of passerine birds, making them the hosts of the virus. When these birds are infected, the virus amplifies, potentially infecting multiple mosquitoes that feed on its blood. These infected mosquitoes may go on to further transmit the virus to more birds. If the mosquito is unable to find its preferred food source, it will choose another. Human blood is sometimes consumed, but since the West Nile virus does not replicate that well in mammals, humans are considered a dead-end host. In humans Person-to-person transmission of<unk> es is not common, but can occur. Blood transfusions, organ transplantation, and the use of blood products can transmit<unk> es if the virus is present in the donor's blood or organs. Because of this, blood and organs are often screened for viruses before being administered. Rarely, vertical transmission, or mother-to-child transmission, has been observed in infected pregnant and breastfeeding women. Exposure to used needles may also transmit<unk> es if they have been used by an infected person or animal. This puts intravenous drug users and healthcare workers at risk for infection in regions where the<unk>  may be spreading in human populations. Virology<unk> es are a polyphyletic group, belonging to various viral genera and therefore exhibiting different virologic characteristics.<unk>  Genome type Genome length Diameter Capsid shape Enveloped? Viral entry Replication site Viral shedding Infected cell(s) Genetic variability African swine fever virus dsDNA 170-190 kilobases ~200 nm Icosahedral Yes Endocytosis Nucleus Budding Endothelial cells and red and white blood cells 22 genotypes Chikungunya virus (CHIKV) +ssRNA 11.6 kilobases 60 - 70 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Epithelial cells, endothelial cells, primary fibroblasts and macrophages Three genotypes Dengue virus +ssRNA ~11,000 nucleobases ~50 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Langerhans and white blood cells Four serotypes Japanese encephalitis virus +ssRNA ~11,000 nucleobases ~50 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Five genotypes Rift Valley fever virus -ssRNA Spherical Yes Cell cytoplasm Budding None Tick-borne encephalitis virus +ssRNA ~11,000 nucleobases 40-50 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Neural cells Five genotypes West Nile virus +ssRNA ~11,000 nucleobases (11-12 kilo bases) 45-50 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Yellow fever virus +ssRNA ~11,000 nucleobases 40-60 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Hepatocytes and white blood cells Zika virus +ssRNA 10794 nucleobases 40 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding
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