What I learned from the video game "Pandemic 2" is that the longer your disease remains undetected, the more widely it can spread; if you're asymptomatic and apparently harmless, you may be able to infect every human before you activate the bad side effects. (Which is the goal of that game.)
In other words, any sign of symptoms would cause Madagascar to remain the last place inhibited by humans since they would close their ports/airports real quick
