Chapter -01: IntroductionMosquito borne diseases are thriving world-wide against centuries of control endeavors. Several important diseases including yellow fever, encephalitis, malaria, filariasis, dengue, chikungunya and O’nyong’nyong(ONNV) fever, known to be transmitted via mosquito vectors, have come out as epidemics concurrently in the history of humankind, and caused billions of death and sufferings by occurring hemorrhagic fever 1, 2. So, it is one of the major concerns to understand the characteristics, immunology and viral transmission processes of this arthropod. Mosquitoes, small midge-like hematophagous dipteran flies belonging to Culicidae family, are spreading world-wide concern because of their invasive nature of female trait causing detrimental effect on mammals, economy and natural biodiversity 3, 4. It is important to note that, among over 3000 species of mosquitoes, only few genera like Anopheles, Culex ,Aedes etc. are potential for transmitting viral infection to human and other mammals. 5, 6 Historically, Anopheles has known to be the vector for malaria fever, occurring dramatic emergence and reemergence incidents over tropical and pacific regions in several times 7-9. It is also have been reported to spread ONNV and West nile virus in Africa and China.10 Culex mosquitoes are competent vectors for encephalitis spreading, transmitting several pathogens like West Nile virus (WNV), western equine encephalomyelitis virus, St. Louis encephalitis virus, Ockelbo virus, bluetongue virus and more, causing hemorrhagic diseases 11-13.From the invasive mosquito genera, Aedes presents the most implicit threat toward human population, causing numerous sporadic viral infections. Most of their species originated mainly from Africa, but spread throughout the world except Antarctica. They can be visually distinguished of distinct black and white markings on their body and legs. Two most prominent species are Aedes albopictus and Aedes aegypti transmitting viruses like Dengue virus, Chikungunya virus(CHIKV), Zika virus, Yellow fever virus, WNV, ONNV and more, following massive deaths 13, 14. Viruses that are known to be spread by arthropods are known as arboviruses, and most of them have come from Flaviviridae and Togaviridae family, uses Aedes spp. as their vector to infect human.15 Some of the flaviviruses(Flaviviridae) are yellow fever virus (YFV) dengue virus (DENV),West Nile virus (WNV) and some of the alphaviruses (Togaviridae) such as chikungunya virus (CHIKV), o’nyong-nyong virus (ONNV), Semliki Forest virus (SFV),Sindbis virus (SINV);and bunyaviruses (Bunyaviridae) such as Rift Valley fever virus (RVFV). These viruses were not known until modern medicine usages were established.16 Flaviviruses are molecularly charachterized as RNA virus (ssRNA), uses mammals as their second host, and transmit by zoonosis and bite, spread through epithelial cell layers of kidney, skin, intestine, testes and more. They replicate in cytoplasm of host cell, and release from secretary parts of host 17. Unlike them, alphavirus genome is monopartite. But pathogenesis of those two famiies of viruses is quite similar in nature. On the basis of serological cross-reactivity, alphaviruses can be divided into seven complexes, among which, Semliki Forest Virus complex (SF complex), composed of 11 viruses like CHIKV,ONNV, Ross River virus, Barmah Forest virus, Mayaro, Semliki Forest virus and some more, characterized by occurring extremely severe headache, myalgia and arthralgia 18-20. In distribution, this clad of viruses are Old World, and now they are being found in the New world. So, such hypothesis exists that may be this group of viruses originated from either from Old World or New World, and distributed to other hemisphere around 2,000 to 3,000 years ago, and, with several mutations in their information pathway, they got increased or decreased level of virulence 21. Among SF group, recently Chikungunya have draw attraction of public health and researchers because of its concurrent epidemics throughout the world, especially in South East Asia.It has been reported that, explosive epidemics were found in 2005–2007 time period on the Indian Ocean islands and in India, with a reemergence history with African, europian and American nations too, from earlier half of 20th century to this one 22, 23. In Bangladesh, this epidemic was first identified in 2008, and then occurred in 2013,2014,2015, and recently in 2017 with greater range 24-26. So, it has got a great importance for extensive study on characteristics of Aedes-CHIKV interaction and the cause of this recent outbreak for effective solution to the loses due to this sporadic event. And it is also necessary to assume about next epidemic event related with mosquito carried pathogens, for having a strong base for risk assessment and epidemic management. This article will give a brief description upon those sectors.