"When you look at a typical influenza virus, for example seasonal flu, where you have millions and millions of people infected, the mortality is less than one percent, a fraction of a percent. The mortality for this if you do the math is 50 cases and you have 30 deaths, so you are talking about a 60 percent mortality already," said Fauci.

The virus has been found in Middle East countries and isolated cases have been exported to Europe by visitors. But the largest cluster of infections is in Saudi Arabia, home to the Muslim holy cities of Mecca and Medina, which draw millions of pilgrims a year. The World Health Organization says it is closely monitoring the situation but is not currently recommending any travel or trade restrictions.