Avian flu is a term used to describe influenza viruses that primarily affect birds. These viruses are highly contagious and can cause severe illness in both domestic and wild birds, resulting in high mortality rates among infected populations. In some cases, avian flu viruses have also been known to infect humans, causing a variety of symptoms ranging from mild respiratory illnesses to more severe conditions such as pneumonia or even death. When human infections occur, they are typically the result of direct contact with infected birds or their droppings, although there have also been instances where avian flu viruses have spread through contaminated environments or via contact with contaminated objects.