What is a Pandemic?

The World Health Organization (WHO) says the outbreak of swine flu in North America has "pandemic potential" because it can spread from human to human.

What flu pandemics are and how they might be stopped or slowed down:

— New forms of flu virus often appear first in livestock, particularly poultry and pigs. The vast majority of these animal viruses do not cross over to humans.

— Flu pandemics occur when a strain of the flu virus mutates into a new form that can spread from human to human and to which people have no natural immunity.

— The new strain will most likely spread through the air, by coughing and sneezing in the same way as seasonal flu.

— Because there is no widespread immunity to the new strain, its effects are worse than normal flu. It is also difficult to predict which age groups will be worst hit by a pandemic strain.

— Until now, scientists have been most concerned the H5N1 bird flu would mutate into a pandemic strain. Since 2003, over 100 people have contracted a deadly strain of H5N1 and more than half of them have died. So far this strain does not appear to spread easily from human to human.

— Flu pandemics usually occur a couple of times each century, but the timing is unpredictable. Serious pandemics occurred in 1918 (Spanish influenza); 1957 (Asian influenza); and 1968 (Hong Kong influenza). According to WHO, the 1918 pandemic killed between 40 million and 50 million people worldwide, making it one of the deadliest epidemics in human history.

— Once a full-scale pandemic breaks out, WHO will raise its alert level to phase six.

— Countries may take measures such as border closures and travel restrictions to delay arrival of the pandemic, but would probably not be able to stop it.

— Widespread air travel means a pandemic would likely circle the globe in the space of three months, according to WHO.

— Most countries would not have enough supplies of anti-viral drugs to treat the entire population. A vaccine would also not be available immediately, and production of sufficient quantities would take some time. Rich countries would likely fare better than poor because they are able to buy large quantities of precious medicines at asking price.

— One of the biggest problems during a pandemic would be treating the large numbers of people who fall ill. Hospital staff would also be affected, reducing the number of health workers able to treat patients. For this reason many countries have declared that health workers would be the first to receive anti-virals.

— Death rates during a pandemic depend on the number of people infected, the vulnerability of the population, how lethal the virus is, and measures taken to stop the outbreak. WHO estimates that a pandemic would kill between 2 million to 7.4 million people, but these figures are based on modeling studies and are not considered precise.

— Aside from the health effects, economic damage would likely be one of the worst effects of a pandemic as workers stay at home and consumption and trade plummet.