What is the avian influenza virus?

This is a virus that has been circulating among birds and typically does not infect human beings. There are, however, three main sub-types of this virus that are known to infect both birds and humans. These are avian influenza A H5, H7 and H9. Nine potential sub-types are known for each of these three, i.e. 27 in all.
Of these 27 known sub-types, so far only five have infected humans. Among them, avian influenza A (H5N1) is the most well known. It is circulating among poultry in as many as 15 countries. In the past, H7N2, H7N3, H7N7 and H9N2 have also infected human beings, causing effects ranging from mild illness to death.