As obligate intracellular pathogens, viruses are the simplest and most intimate of the various life forms (bacteria, fungi, worms, etc.) that are programmed to live in, or on, other organisms. Although some viruses can survive for a time in the external environment, they all ultimately rely on strategies requiring further infection and replication in naive hosts that allow high levels of virus production to facilitate transmission. At the same time, they must at all times cope with the host immune response and therefore develop counter-mechanisms to guarantee their survival.

Insect viruses have probably existed for as long as insects themselves, and have long been of interest to humans. Viral infection of insects can have serious consequences. For example, yellow fever virus, West Nile, and Dengue viruses are mosquito-borne and cause severe illness, such as encephalitis in humans (Mackenzie et al., 2004). As exemplified in recent years for the West Nile virus in the USA, these viruses provide a representative example of emerging diseases of global significance. In addition, many insect or invertebrate viruses are of great concern, as they cause substantial damage to the environment or agriculture. For example, some of these viruses threaten beneficial insects, such as honey bees, or human enterprises, such as the silkworm industry. White spot syndrome virus is also one of the most virulent pathogens of cultured shrimps and causes massive loss of this commercial product worldwide. Therefore, a genetic model for studying host-virus interactions in insects would be beneficial to society in many ways.

The fruit fly, Drosophila melanogaster has been established as a powerful model for the mechanistic studies of innate immunity (see Chapter 2). Most studies so far have focused on the response of Drosophila to extracellular pathogens, such as bacteria and fungi, and have uncovered two complementary pathways, Toll and Imd, regulating different members of the nuclear factor kB (NF-kB) family of transcription factors, and expression of genes encoding antimicrobial peptides (Lemaitre and Hoffmann, 2007). In contrast to the wealth of information now available for fungal and bacterial infections, the interaction of Drosophila with viruses has only recently started to be addressed. Here we review the progress made in the past few years on antiviral defence mechanisms in Drosophila, and discuss the relevance of these findings for our understanding of the complex interaction of viruses with their invertebrate or mammalian hosts.

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