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Parasitic protozoans and helminths that are transmitted from host to host by haematophagous insects have complex life cycles that often require a period of development (the pre-patent period), and sometimes asexual multiplication, in the host or vector before transmission can occur. For transmission to be successful the parasite must be in a suitable location and at a mature stage when the vector makes contact with the host. Biting insects are attracted to their hosts by the carbon dioxide, volatile chemicals and heat they emit and by their appearance. However, blood feeding is a risky foraging strategy and feeding decisions must be made to avoid host defence reactions. Using examples of parasites that infect humans causing tropical diseases such as malaria, leishmaniasis and filariasis, experimental evidence will be discussed that suggests that the attractiveness of infected hosts to the parasite’s vector changes specifically when transmission stages are present. Likewise, evidence will be presented that shows that vectors infected with patent infections alter their feeding persistence and probing behaviour in ways that may enhance parasite transmission. Several parasites are also known to cause a reduction in vector fecundity and may alter the trade-off between reproduction and longevity, thus enhancing the prospects of transmission. The adaptive nature of these infection induced behavioural and physiological changes in host and vector will be discussed.<p><p>Keywords: