There was a lot of exploration of 'uncharted territories' in the tropics inthe 19th century by highly-mobile Europeans, so it's possible - an explorer up the Congo/Amazon/Irrawady picks up a flu bug that incubates as he returns to Cape Town/Rio/Singapore. The highly infectious disease finds a home in the crowded slums around the port, and then catches a ride on merchant and war-ships that are heading to all parts of the world. Ports make excellent disease propagation sites, being dirty, wet, and crowded.
You need to provide more information on the epidemology of this uber-plague to say what the effects would be. From the on-set of symptoms, is death quick or prolonged (think Ebola v. AIDS)? How debilitating during the progression? What is the transmission vector (fluid, air borne, rats)? How robust is the virus to temperature/environment changes (Could Scandanavia be spared because cold kills the virus?). How long before the plague burns itself out,allowing interactions again?
If the disease is quick, then it would be devastating to towns and cities. There would be a much greater impact on population centers than on the countryside - farmers may easily be spared, if their contact with others is infreqeunt. A more prolonged disease would mean greater chance of the infection spreading to more remote areas. An emptying of the cities could occur, with survivors abandoning them in favour of refuges in the country. Could set back the migration of humanity from rural to urban by many cetnturies.
If the plague runs its course quickly through the population, leaving immune survivors that can converse without it being a death sentence in a fairly short period of time, then countries may not collapse, but economies will. A step-back to medival economies, with local production for local use on farmsteads and small villages is possible. The specialization of skills that industrialization and cities provided would be lost, much as happened with the decline of Roman cities and populations a thousand years before. Psychologically there might be a huge increase in distrust of foreigners, bringing cross-border trade to a halt as well.
Politically there would of course be great upheaval, but I could see any surviving aristocracy (with the money and power that comes with it) reasserting their traditional roles of overseeing and adminstering the local area - a return to Feudalism bascially. Whether the liberalism of the Renaissance would be seen as a bad example and a reason to keep the peasants on the farm or not is hard to say.
Militarily this would be disaster. For one, barracks being also crowded and unhygenic, the toll on soldiers would be far higher than the general populace. Two, much of the surviving military might will be used to enforce the will of the surviving power structure from the chaos during the transistion. There certainly would be no projection of power over-seas, and colonies would be abandoned to their own devices. It would be come a much 'larger' world again