Now a two-year-old Cambodian girl is in a serious condition in Phnom Penh after being hospitalized with the H5N1 virus, also known as avian, or bird, flu.

Sonny Inbaraj Krishnan, the communications officer for the World Health Organization in Phnom Penh, says the development has health professionals concerned.

“This is the fourth case this month of human influenza H5N1," Krishnan said. "Last year we had three cases, so within one month in the new year we've got four cases, and we're quite concerned about that.”

H5N1, which can spread from infected poultry to people, was first detected in humans in 1997 in Hong Kong.