This manipulation of the host dermal microenvironment by infected arthropods also provides an advantage to microorganisms by creating an optimum environment for their transmission and the establishment of infection (Figure 3.1). This is exemplified by the persistence of infectious agents at the site of injection by ticks, which permits the infection of naïve ticks in the absence of systemic infection of the host (‘salivaactivated transmission’) (Figure 3.2). Additionally, the presence of organisms within an arthropod may modify its behaviour. For example, Borrelia-infected ticks have altered questing behaviour, which provides them with an advantage in terms of acquiring their target host. In contrast, Leishmania-infected sand flies also have modified feeding behaviour, but here the advantage is to the microorganism rather than the arthropod. The plug of Leishmania parasites within the proventriculus interferes with the intake of blood, making it necessary for the sandfly to probe the host dermis more frequently.