FACTBOX-New flu still raises questions among experts

By Maggie Fox, Health and Science Editor

WASHINGTON, Oct 15 (Reuters) - How many people have been infected with the new swine flu? How many will it kill? How do you catch it?

Health authorities around the world are trying to persuade people to line up to be vaccinated against the pandemic H1N1 virus even as they struggle to answer these basic questions.

H1N1 swine flu was declared a pandemic in June and the World Health Organization says more than 375,000 people globally have been infected and 4,500 have died.

But these are just laboratory-confirmed cases and the WHO says they are the tip of the iceberg. Only a small fraction of people who get swine flu ever have it confirmed by a doctor.

Here is a rundown of what experts do and do not know about pandemic H1N1 influenza.

WHAT THEY KNOW

* The virus spread around the world in the space of six weeks and is now virtually everywhere.

* It strongly resembles seasonal influenza in that it causes fever, sore throat, a dry cough, muscle aches and tiredness.

* Unlike seasonal influenza, worst among the elderly, this new influenza batters younger people.

* Many people escape without symptoms. Dr. Tim Uyeki, a flu expert at the U.S. Centers for Disease Control and Prevention, says studies show 12 percent of people do not have fever. "A lot of people might think they don't have influenza because they don't have classic influenza-like illness," Uyeki told a meeting of the Pan American Health Organization on Wednesday.

* H1N1 may cause symptoms not normally seen in seasonal flu. "We are seeing diarrhea and vomiting in children of all ages as well as adults," Uyeki said. It can also infect the heart, a condition called myocarditis, affect the brain in what is called encephalopathy and cause multiple organ failure.

* The incubation period -- the time from infection to when symptoms begin -- ranges from one to four days.

* People can spread virus for 24 hours after the fever stops.

* Adults over the age of 60 are less likely to be infected and to become seriously ill with H1N1, in part because they seem to have some immunity to it. The highest rates of hospitalization are among the youngest children -- aged up to 4 -- but the highest rates of death are in adults aged 50 to 64.

* The new vaccine should protect against H1N1 well and antiviral drugs can treat it if given soon enough.