WASHINGTON (Reuters) - The world has moved closer to the threat of a pandemic of a new kind of flu, with 149 people suspected to have died from it in Mexico and new cases being detected around the globe.

Just how bad is this new flu strain, how far will it go and how long will the outbreak last?

Here are some questions and answers about the outbreak:

HOW MANY PEOPLE HAVE DIED? HOW MANY ARE INFECTED?

All deaths so far have been in Mexico, where 20 of the 149 reported fatalities have been confirmed to be from the H1N1 swine flu virus. There are 1,600 suspected cases in Mexico and 64 confirmed cases in the United States, and a few cases in Canada, New Zealand, Britain, Spain and Israel.

WHY ARE THERE ONLY DEATHS IN MEXICO?

No one is sure. It is important to remember that health officials are now taking a snapshot of the past -- they are not reporting on new infections at this point, just tracking down old infections and they are only finding them where they are looking. The Mexican authorities looked in hospitals, where serious cases will, of course, be found. U.S. health officials found their cases during routine screening of people with flu-like symptoms, most in walk-in clinics, so they have naturally found milder cases.

Influenza experts say they fully expect to find deaths in other places, including the United States and elsewhere, as the search goes on. One problem is that people die of respiratory diseases regularly and the cause is often not determined.

WHY WOULD IT KILL SOME AND NOT OTHERS?

Seasonal influenza kills 250,000 to 500,000 people every year in a normal year and all sorts of factors determine who dies. Elderly people often die but sometimes perfectly healthy adults and children die. Sometimes flu makes people susceptible to bacterial infections, called secondary infections, and if the virus and the bacteria are circulating at the same time in the same place there can be clusters of deaths.

WHAT KIND OF FLU IS IT AND HOW IS IT SPREADING?

The virus is an influenza A virus, carrying the designation H1N1, but it contains DNA from avian, swine and human H1N1 viruses. It appears to have evolved the ability to pass easily from one person to another, unlike most swine H1N1 viruses which only very occasionally infect people and usually only infect one person and then stop there.

Flu viruses are all passed on by sneezing, coughing or when people pick up the virus on their hands. This one likely originated in pigs, but the Mexican government and the World Health Organisation have ruled out any risk of infection from eating pork.

HOW SERIOUS IS IT?

The Geneva-based WHO has declared the flu a “public health emergency of international concern” and raised the threat level for a pandemic, a global epidemic of new disease. H1N1 swine flu poses the biggest risk of a large-scale pandemic since avian flu re-emerged in 2003, killing 257 out of 421 infected in 15 countries.

It is not clear yet whether this virus could actually become a pandemic.

HOW IS THIS FLU DIFFERENT FROM ORDINARY FLU?

The swine flu is characterized by common flu symptoms -- sudden fever, muscle aches, sore throat and dry cough -- but may cause more severe vomiting and diarrhoea.

New flu strains can spread fast because no one has natural immunity and a vaccine can take months to develop. This strain is confusing because it is an H1N1 -- a type that has been around since the 1918 “Spanish Flu” pandemic that killed at least 40 million people globally.

Usually if a new flu strain is related to one that has been around for years, people have some immunity and they no longer can cause pandemics. But this new strain has taken on genetic elements from animal viruses, and this may be genetically unique enough to pose a pandemic threat.

HOW BAD COULD IT GET?

A 1968 a “Hong Kong” flu pandemic killed about one million people globally. The 1957-58 pandemic killed about 2 million. The 1918 pandemic killed between 40 million and 100 million, according to some estimates. However, the WHO says the world is now better prepared to withstand a flu pandemic. Vaccines and antiviral drugs are available that were not around during previous pandemics.

In 1918, there was a first wave of mild flu in April. It then seemed to disappear during the Northern Hemisphere’s summer but came back severely in August. WHO and CDC officials say it is possible this virus could behave in the same way, or completely differently.

ARE THERE ENOUGH DRUGS AND VACCINES?

Most countries have been stockpiling supplies of two antiviral drugs -- Tamiflu, known generically as oseltamivir and made by Roche AG and Gilead Sciences Inc; as well as Relenza, known generically as zanamivir and made by GlaxoSmithKline and Australia’s Biota Inc..

A third company, BioCryst Inc. is working to licence its experimental flu drug peramivir.

Older flu drugs called amantadine and rimantadine do not work very well any longer against any influenza strain except sometimes in combination with newer drugs.

There is no vaccine yet against this new strain and health officials say the seasonal flu vaccine is unlikely to provide any protection against it. The CDC and WHO are working with companies to start making a new vaccine if it is needed, but the process takes months.

WHAT CAN I DO?

Wash your hands. It is proven to be the best way to protect against infection with a range of germs, including flu. Experts generally agree that face masks, especially the surgical masks now seen on the street of Mexico City, offer very little protection. Flu viruses can float on little particles of spit or mucus, in general no further than three to six feet (1 to 2 metres), but they then settle on surfaces and can be transferred to the mouth, eyes or nose.