As an Indian I can tell you even a few infected people can come into the country and cause an outbreak here.
Its not like in US, the sewage infrastructure in India is very bad. There are open air sewage drains, mosquito infestation is every day life and in cities people live in very densely crowded homes. All of this contributes to a rapid spread in case of a outbreak.
In fact dengue outbreaks are common in India every summer, or a few months.
There's a good point. The number of visitors from those countries actually remained typical or declined in London for 2012. It is conceivable that the increased hotel and airfare costs actually deter travel from poorer countries.