In defining a pandemic, a closely-related term, epidemic, must be defined first. An epidemic occurs when new cases of a certain disease occur in a given human population, during a given period, exceed the normal rate of new cases over a specific period of time. Defining an epidemic is subjective, depending on how much is perceived to be the "normal" rate of new cases per a specific period of time. Due to this subjectiveness, a few cases of rare diseases may be classified as an epidemic while many cases of a common disease, e.g. common cold, would not.
A pandemic is an epidemic of an infectious disease that spreads through populations of a large region, for instance a continent, or even worldwide. According to the World Health Organisation (WHO), a pandemic is announced when three conditions have been met:
- Emergence of a disease new to a population.
- Agents infect humans, causing serious ilness.
- Agents spread easily and sustainably among humans.
It is important to note that a disease may only be considered pandemic if it is infectious. A disease such as cancer, which causes a large number of deaths worldwide, may not be called a pandemic because it is not infectious or contagious. Pandemics often trigger a global action, usually regulated by the WHO. For example, the 1955 Global Malaria Eradication Campaign which involved spraying insecticides in designated areas of the world such as the United States and India.
The world is currently worried about the recent outbreak of the Influenza A (H1N1) virus or more commonly known as the "swine flu." One of the worst pandemics in history, the 1918 Spanish flu pandemic, was also caused by a strain of the Influenza A (H1N1) virus. It is estimated that the the pandemic killed 50 to 100 million people worldwide. There is a fear that the swine flu may also possess pandemic potential, thus individual safety measures need to be taken immediately.
Considering that a pandemic usually starts suddenly and ends suddenly, a pandemic or an epidemic is certainly a reminder by Allah to His servants who are always forgetting Him; and as a reward for the wrongdoers.
We shall test your steadfastness with fear and hunger, with loss of life and property and crops. Give good news to those who endure with fortitude, who in adversity say: 'We belong to God, and to Him we shall return.' God's blessing and mercy will be upon them; they are rightly guided. (Al-Baqarah: 155-157)
Reference: Wikipedia and other sources.