Influenza A (H7N9) virus. TEM, negative stain. On April 1, 2013, the World Health Organization (WHO) reported the first human infections with this strain in China. H7N9 is an avian (bird) influenza (flu) virus. Human infections with avian influenza (AI, or “bird flu”) are rare but have occurred in the past, most commonly after exposure to infected poultry. However, this was the first time that this H7N9 flu subtype had been found in people. This virus is very different from other H7N9 viruses previously found in birds.