Most affected areas are 3rd world countries unable to provide adequate health care, eradication methods and programs to combat the spread of mosquito borne disease.
Presence of containers of stored water, discarded junk and dams increase the breeding grounds which are available to mosquitoes
Unplanned urbanisation (where there is typically lack of waste removal infrastructure) also facilitates breeding mosquitoes for this reason
Globalisation: increased movement of people over large geographical areas in shorter times, increases the distribution of mosquito borne diseases as exposed to new pathogens → no immunity to these diseases