According to the latest official data, H7N9 avian influenza has infected 130 people in China, and killed 35, since it was found in humans for the first time in March.
"Influenza viruses constantly reinvent themselves. No one can predict the future course of this outbreak," Margaret Chan said, but added that "although the source of human infection with the virus is not yet fully understood, the number of new cases dropped dramatically following the closing of (China's) live poultry markets."
"At present, human-to-human transmission of the virus is negligible," she said in her address to some 3,000 delegates attending the 66th World Health Assembly in Geneva.
Chan also thanked China for its close collaboration with WHO in sharing its information on the situation and for having "promptly traced, monitored, and tested thousands of patient contacts".
Chan also talked about the deadly SARS-like coronavirus, detected for the first time in the Middle East last year. "To date, 41 cases, including 20 deaths, have been reported," she said, adding that "though the number of cases remains small, limited human-to-human transmission has occurred and health care workers have been infected."
The virus is a cousin of Severe Acute Respiratory Syndrome (SARS), which triggered a scare 10 years ago when it erupted in east Asia, leaping to humans from animal hosts and eventually killing some 800 people.
"These two new diseases remind us that the threat from emerging and epidemic-prone diseases is ever-present. Constant mutation and adaptation are the survival mechanisms of the microbial world. It will always deliver surprises."