A pandemic is a global outbreak of a serious disease that exceeds the "normal" levels of mortality and infection levels for typical disease. In this regard, the definition of a pandemic is partially subjective, but in most cases, the difference between the normal background state and a pandemic state is strikingly obvious. Throughout history, pandemics have been the worst killers of human beings, killing more people than all wars and accidents in modern history combined. Only in the last 90 years or so does it seem that historical pandemics are mostly under control, although the HIV virus, which emerged in the 1980s, has killed millions, and continues to be a serious problem to this day.
Historically, the largest pandemics have been due to smallpox, influenza, cholera, typhus, and bubonic plague. The greatest pandemic in history is often cited as the Black Death (thought to be bubonic plague), which claimed 75 million lives and killed about a third of the population of Europe and China in the late 1340s. Recovery took more than a generation. The native Americans and native Australians were fortunate that these populations were disconnected from those of Africa and Eurasia at the time. The Black Death was spread by infected rats.
Another massive pandemic, the Spanish flu, spread worldwide in 1918, immediately after WWI, killing 50-100 million people and rivaling the intensity of the Black Death. This flu was unusual in that it killed people in the prime of their lives rather than just the old or infants. Deaths are thought to have been caused by cytokine storms, a hyperactive immune system response that leads to the death of the organism it is meant to protect. In a cytokine storm, immune cells clog the body so rapidly that they do even more damage than the flu itself.
When they occur, pandemics have radically altered the course of history and changed world events. If the Black Death had not occurred in Europe, economic and technological progress might have been accelerated and Columbus may have reached the Americas as much as a hundred years earlier. Today, many governments fear a new pandemic caused by a mutation of the avian flu, and hundreds of millions of doses of immunization shots have been stockpiled.