A pandemic is defined as a situation when a new strain of a disease (such as the current H1N1 flu virus) emerges world wide in human populations and large number of people are infected at the same time. This differs from an epidemic which is widespread infection of people by a known virus, or a minor variation of a known virus.
The first major influenze pandemic was the Spanish Flu outbreak of 1918, which is actually believed to have originated among American soldiers stationed at Ft. Riley, Kansas, which killed tens of thousands of people world wide. We have also had flu pandemics in 1957 and 1968. Flu epidemics kill around 36,000 people in the United States each year. Most of the victims, however, are the elderly or babies or people with weakened immune systems. A pandemic, on the other hand, because it is a new strain against which people have not developed any immunity, attacks otherwise healthy populations, and puts a strain on healthcare systems and economies.
An influenze pandemic is thought to come in two to three waves, each lasting from 6 to 8 weeks. Infected individuals are contagious the day before symptoms are present and remain contagious for up to five days after symptoms start. In a pandemic, because the disease is new and no immunities are present, many people are infected at the same time, and with air travel and crowded conditions, the disease spreads rapidly. Study of the pandemics of 1918, 1957 and 1968 indicate that a pandemic will last from 14 to 16 weeks. These are, however, only estimates. The pandemic of 1918 reportedly lasted nearly a year before it finally died out.
Contrary to popular belief, pandemics don't last until the last infected person is dead. The flu virus is one of the most adaptable of the viral ailments and it is constantly mutating. This explains why different vaccines have to be developed each year to cope with the annual epidemic outbreaks. A virus that killed every host it infected, however, would turn the laws of evolution on their collective ears. The pandemic lasts until the surviving hosts develop sufficient antibodies to the new virus, and then it hunkers down and begins the process of transforming all over again. Even the Black Death, while it killed millions across Europe, didn't kill everyone. A virus that kills 100% of its hosts is committing suicide and this is contrary to the way nature has designed living creatures - even viruses.
We have not yet reached the pandemic state with H1N1, and everyone is praying that we do not. With the speed that people can move from one country to another currently, and the larger, more dense populations in some countries, a pandemic today could have a much more devastating effect than that of 1918. The edge we enjoy today is a greater understanding of how diseases spread, and international organizations and agreements to cope with global infections.