Madagascar is one of those places where the Bubonic Plague still lives. there's several 'hotspots' around the world, including California...
But this outbreak isn't typical. For one it is in the big cities, not isolated rural areas. And two it has changed into the Pneumonic version, which is airborne.
Madagascans help the plague spread with some of their burial rituals, like 'dancing with the dead'. This lets the fleas which actually carry the Plague spread to living hosts.
And anyone who knows the game Pandemic remembers that Madagascar is usually the last place to fall. If your virus starts there you have a good chance to win! And destroy humanity.