The WHO says the contagiousness of the virus will influence the speed of spread, both within countries and internationally and this, too, can influence severity, as very rapid spread can undermine the capacity of governments and health services to cope.

People with underlying chronic conditions, such as cardiovascular disease, hypertension, asthma, diabetes, rheumatoid arthritis, and several others, are more vulnerable as they are more likely to experience severe or lethal infections.

Pandemics also tend to encircle the globe in at least two and sometimes three, waves and the severity of subsequent waves can differ dramatically in most countries.

Influenza viruses also mutate frequently and unpredictably and the emergence of an inherently more virulent virus during the course of a pandemic is always possible - the 1918 pandemic began mild and returned, within six months, in a much more lethal form.

The same virus that causes only mild symptoms in countries with strong health systems can be devastating in other countries where health systems are weak, supplies of medicines, including antibiotics, are limited or frequently interrupted, and hospitals are crowded, poorly equipped, and under-staffed.

The WHO says the H1N1 virus is a new virus that has not been seen previously in either humans or animals and scientists suspect that pre-existing immunity to the virus will be low or non-existent, or largely confined to older population groups.

H1N1 appears to be more contagious than seasonal influenza and apart from the outbreak in Mexico, which the WHO says is still not fully understood, the H1N1 virus tends to cause very mild illness in otherwise healthy people and nearly all cases of illness, and all deaths, have been detected in people with underlying chronic conditions and this is of particular concern.

Scientists are concerned about possible changes that could take place as the virus spreads to the southern hemisphere and encounters currently circulating human viruses as the normal influenza season in that hemisphere begins.

The fact that the H5N1 avian influenza virus (bird flu) is firmly established in poultry in some parts of the world is another worry because it is impossible to predict how the H5N1 virus will behave under the pressure of a pandemic.

At present, H5N1 is an animal virus that does not spread easily to humans and only very rarely transmits directly from one person to another.

H1N1 flu spreads in the same way that regular seasonal influenza viruses spread - mainly through the coughs and sneezes of people who are sick with the virus.