Avian Influenza

Influenza A H7N9

A transmission electron microscope (TEM) image of influenza A H7N9.

A Past and Present Threat

In December 1997, a team of CDC scientists aided in the investigation of an epidemic caused by a novel influenza virus in Hong Kong. An influenza virus previously only seen in fowl (H5N1) had infected 18 people and killed six. The robust investigation concluded that close contact with live poultry caused the human infections. To contain the spread of the epidemic, millions of poultry were killed.

H5N1 viruses re-emerged in 2003 in poultry, and since that time, have caused sporadic human infections, first in Asia, and later in Africa, Europe, and the Middle East.

In 2013, another novel influenza virus with avian origins (H7N9) emerged to cause human illness across multiple provinces of China. Within two months, the number of human H7N9 infections equaled the number of human H5N1 infections that had been reported over ten years. CDC worked closely with China CDC to respond to this emerging public health threat. While human infections with H7N9 have declined in recent years, to this day H7N9 remains the number one threat for a potentially severe influenza pandemic.

Avian Influenza