Greece sneezed, and now most of Europe has a cold. The European debt crisis has already spread like a virus from Greece to Ireland and Portugal, and other countries are now at risk: Spain, and Italy are probable candidates for financial problems.

Contagion also has much to do with actual economic links among countries. Researchers have identified financial ties in particular as responsible for the “fast and furious” spread of crisis from one country to another. Trading activity between countries, however, can propagate economic sickness more slowly.