Q+A-How serious is swine flu? How bad could it get?

WASHINGTON, April 28 (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?