THE images are frightening. Mexico City, one of the largest cities in the world, is eerily deserted, because of growing fears that a form of flu that has jumped species from pig to human could be the start of a killer world epidemic, or "pandemic".

The handful of Mexicans venturing out are wearing face masks to try to protect themselves from a contagion that is believed to have killed more than 100 people. The first death in Mexico was on April 13 and, since then, the toll has risen steadily. There are also about 1,600 suspected cases in the country.

There are also suspected cases in the US, Canada, New Zealand and Spain.

While anti-viral drugs such as Tamiflu appear to be effective if given early enough, there is something about the new outbreak that worries experts.

Like the devastating Spanish flu pandemic of 1918, the new outbreak appears to target younger people. The authorities in Mexico say most of the dead are aged 20 to 40 -and not the very elderly that tend to die of ordinary flu in the UK.

In the North-East, the Health Protection Agency (HPA) is monitoring the situation and working closely with the North-East Strategic Health Authority. In North Yorkshire, the area is covered by the HPA in Leeds and the Yorkshire and Humber Strategic Health Authority.

Earlier this week, Professor Nick Phin, from the HPA's pandemic unit, said he did not see how the UK could avoid the outbreak if it continued to spread.

Officials at the HPA were working on the basis that up to half the people in Britainmight contract swine flu.

For the last few years every NHS organisation in the region, from hospitals and ambulance services to primary care trusts, has had to draw up contingency plans in the event of a pandemic being declared by the Government.

These plans have been tested at regular intervals.

One of the most sinister aspects is the plan to extend the capacity of hospital mortuaries in the region - if it is needed.

Q What is known about swine flu?

A It is a contagious, respiratory disease, caused by influenza type A, which infects pigs. Until now, it has not infected humans, but this latest form has jumped from pig to humans. Worryingly, it can be spread from person to person, probably by sneezing and coughing.

Q What is new about this disease?

A Some of the human cases confirmed so far are suffering from a version of the H1N1 strain of influenza, which has never been seen before.

H1N1 is the same strain of influenza, which causes "normal" flu in humans. But scientists have found that this new version of H1N1 is different. Samples have shown that it contains genetic material found in versions of the virus that affect pigs, birds and humans.

It seems the new version of H1N1 is the result of a mixing of different versions of the virus in the same animal host.

Q Just how dangerous is this new strain of swine flu?

A The symptoms of swine flu in humans seem to be similar to normal flu. These include fever, sore throat, cough, achingmuscles, chills and tiredness.

So far, most of the cases reported around the world appear to be mild, but some people have died after contracting swine flu in Mexico. Experts are concerned that many of the victims in Mexico are relatively young, aged 20 to 40, which suggests that something unusual is happening. Normally, deaths from flu tend to be concentrated among elderly people.

Q Can this virus be contained, or is it too late?

A Now that cases are appearing in different parts of the world, most experts believe that it will be very hard to try to contain the outbreak. The widespread network of international air travel makes this more difficult.

Q Can swine flu be treated?

A According to the US authorities, the two standard anti-viral drugs, Tamiflu and Relenza, seem to be effective at treating cases of swine flu. However, the drugs must be given to patients at an early stage. In the UK, the authorities have stockpiled large quantities of Tamiflu.

Q How worried should we be in the North?

A Whenever a new strain of flu emerges which can pass from person to person, it is monitored very closely by the authorities in case it has the potential to turn into a pandemic.

The health authorities that cover the NorthEast and North Yorkshire are both monitoring the situation, along with the Health Protection Agency, which has offices in Newcastle and Leeds.

Detailed contingency plans have been drawn up so the region's health services are ready to cope with a potential flood of flu victims.

Experts say it is too early to assess the situation fully, but there are concerns that the first major flu pandemic since 1968, which killed about a million people, could be under way.