The avian influenza viruses that keep spreading among birds are constantly changing forms, and new strains of flu develop all the time. Some influenza strains can even cross the species barrier and jump from birds to human beings. Several global flu pandemics have occurred in the past, and if a new avian flu strain acquired the ability to spread from person to person, it would cause a widespread health crisis. However, it is difficult to predict when and how a pandemic outbreak will occur.