The World Health Organization says efforts to prevent the spread of a dangerous virus need to be stepped up ahead of the start of the pilgrimage season to Saudi Arabia.

The vast majority of the 701 confirmed cases and 249 deaths worldwide from Middle East respiratory syndrome have occurred in that country.

Saudi officials recently acknowledged they had under counted the number of cases, prompting concern that the kingdom might not be properly tracking infections.

But WHO's flu chief Dr. Keiji Fukuda says there is "a big amount of improvement" in Saudi Arabia.

Fukuda said Tuesday that pilgrims with underlying conditions are particularly at risk and countries with limited health care systems, such as in Africa, are "vulnerable."

Infections can be halted using simple measures such as hand-washing.