What is a Pandemic ?

Most infectious diseases do not occur at a constant rate with a predictable number of cases per month. With something like Lung cancer it is known that there will be x thousand cases per month in the UK. This level remains fairly constant over time. With an infection, one person can pass the disease to many others, so once thee is a first case it can be expected that many more will follow in that locality. This is termed an "outbreak" of the disease.

If the outbreak spreads so that it affects most or all of the country then it is termed an "Epidemic"

If the epidemic spreads from country to country and then from continent to continent so that much of the world is affected, then it is termed a Pandemic.