In today's health matters, the United Nations issues a warning saying wild bird migration may have brought bird flu back to previously virus-free countries.

The strain of bird flu, known as H5N1, can be resistant to existing vaccines. It is spreading in China and Vietnam, according to the U.N. Food and Agriculture Organization.

Last week the world health organization reported a 6-year-old Cambodian girl died from bird flu earlier this month. She was the 8th person to die from this disease in Cambodia this year.

The virus had been eliminated from most of the 63 countries infected at its peak in 2006. It has remained in Bangladesh, China, Egypt, India, Indonesia and Vietnam.

Avian Influenza affects both wild and domestic birds including ducks, gulls, chicken and turkey. It can spread to people who have extensive contact with infected birds, but not by eating poultry or eggs that have been properly prepared.

In humans, bird flu can cause problems ranging from eye infections to flu-like symptoms and severe respiratory problems.