Scientists in Portugal, led by Gabriela Gomes, came up with a mathematical model to see if there is a threshold between malaria being endemic in a region or not. What they found, after applying the model to 8 Sub-Saharan African states, is that there is such a threshold. Theirs is the first model to take into account asymptomatic infections' role in disease transmission. They hope that it can be used to develop feasible interventions with a numerical value/percentage of mosquito nets that should be handed out or the number of people treated with anti-malarials to make a certain area malaria-free.
Check it out: http://www.sciencedaily.com/releases/2008/03/080311215658.htm