FACTBOX: Flu spreads silently, causes vague symptoms

(Reuters) - A new strain of influenza has killed 103 people in Mexico and spread across North America and possibly as far afield as Spain and New Zealand. No one is certain about how virulent the virus is -- meaning how severe the symptoms are -- or what its patterns of transmission are.

Here are some facts about influenza in general, its symptoms and how it is transmitted.

* "Influenza" refers to a family of viruses that include influenza A, influenza B and influenza C.

* Symptoms include sudden onset of fever, muscle aches, headaches, cough and sometimes diarrhea and vomiting. Flu feels far more severe than a cold.

* People can pass flu along before they feel ill and after they feel better. Flu viruses can be found not only in nasal secretions but in fecal matter.

* The "incubation period" for flu -- the time it takes from infection before symptoms develop -- is usually about 24 to 48 hours, although it is not clear what the case is for this new H1N1 swine flu virus.

* The virus can be transmitted on particles of saliva and mucus when people cough and sneeze close to one another. Experts generally agree that three feet (1 meter) is the distance these particles can travel between people.

* Flu viruses can also live for days or even weeks on dry surfaces. More and more evidence shows that people very frequently infect themselves by touching a contaminated surface, such as a computer keyboard, and then touching the nose, eyes or mouth.

* Flu viruses evolve, or mutate, constantly. New strains emerge regularly, which is why the annual flu vaccine must be reformulated every year.

* Flu viruses are believed all to originate in animals. Little changes or mutations in the DNA can allow them to more easily infect people. Some cause more serious symptoms than others.

* Flu viruses are frequently deadly. The World Health Organization estimates that flu kills 250,000 to 500,000 people in a normal year, more during pandemics, which last occurred in 1968, 1957 and 1918.

* Flu can kill directly by causing pneumonia, and it can also make people more vulnerable to bacterial infections that also kill.

(Reporting by Maggie Fox)