Historically, our planet has experienced several pandemics in the last millennium, all of which killed several million people and all of which left some sort of documentary record. The Black Death of 1346-50, probably a mixture of bubonic plague and anthrax, was introduced to Europe via the overland trade route to East Asia, and slaughtered over one-third of the continent's population; by the end of the 1300s, the global population had fallen by about 25% (or 100 million people). The epidemics of smallpox and other illnesses that European mariners introduced to Mexico, Brazil, and the Caribbean in the 16th century killed several million Indians and probably reduced those regions' indigenous populations by 80% before the century was out (see Crosby, Germs, Seeds, and Animals [M.E. Sharpe, 1994]). Cholera pandemics in the 19th and early 20th centuries killed upwards of 10-15 million people. Spanish flu killed at least 50 million people worldwide at the end of the First World War. HIV/AIDS has killed 30 million people worldwide since the 1980s, and as many more people are still living with the disease today.
History can tell us a great deal about the consequences of a large-scale epidemic. Massive die-offs, first of all, can result in the loosening of economic bonds, as surviving serfs or peasants use their scarcity to negotiate with landlords for freedom, but they can also result in the stark curtailment of freedom. When labor is scarce, lords and conquistadors are just as likely to resort to slavery to provide it. Parts of Europe where slavery was in decline, like Italy, saw a resurgence of slave labor in the decades following the Black Death, and when Spanish and Portuguese colonists found that their Indian laborers were dying they replaced them with African slaves.
Epidemics can lead to medical breakthroughs, like the development of public sanitation to fight cholera and of anti-retroviral drugs to fight AIDS. But they can also provoke superstitious responses and violence: think of Europeans who blamed bubonic plague on the Jews, Huron Indians who blamed smallpox on Jesuit missionaries, and evangelical Christians who attribute AIDS to divine displeasure with homosexuality. And, finally, huge die-offs generally don't have any long-term effects on the survivors. Given the relationship between long-distance travel and large-scale epidemics, one might expect trade and travel to decline after pandemics, but I know of no case where this has occurred. Humans also still prefer to live in crowded cities, sometimes in close proximity to livestock, an ideal environment for generating new illnesses. They tend forget the need for prophylactic measures, like vaccination or condom use, once a generation has passed since the peak of a die-off. A realistic novel or movie set after a future plague would have some of the survivors sold into slavery, or the effective equivalent; others killed as scapegoats; and the rest forgetting anything ever happened – until the next epidemic came along and killed another hundred million people.
What, finally, might our fictional (we hope) future superbug be? Damned if I know, though it's comforting to realize that epidemics are to some degree self-limiting. Truly frightening and lethal diseases like Ebola tend to kill their hosts before they have time to spread; those that are both highly contagious and have a long incubation period, like smallpox or influenza, can spread far and wide but kill only a fraction of those infected with them (30%, in the case of untreated smallpox, less, in the case of flu). There may be exceptions. Extreme drug-resistant tuberculosis sounds like it's both contagious and highly fatal, and apparently one can only treat it with chemotherapy and quarantine, both of which are expensive. It's also apparently possible to create a genetically engineered strain of smallpox that suppresses the host's immune response, making it both highly contagious and 100% fatal. I can't imagine anyone setting a science fiction novel in a post-immunosuppressant-smallpox world, however, because there wouldn't be any humans left to write about…