What needs to happen before the bird flu can be transmitted from human to human?
Scientists maintain that the current H5N1 virus must change, or mutate, in order for transmission to occur easily from human to human. For this to occur, one of two things must happen: 1) the virus must mutate spontaneously to become more virulent for humans (called antigenic drift), or 2) the virus must infect an animal that is also susceptible to human viruses, such as a pig. As both viruses multiply inside the animal, genetic material from the bird flu and the human flu viruses mix. In the process, a new virus that is capable of being transmitted easily among humans, but also has the features of the bird flu virus, emerges and causes a pandemic. This process is called antigenic shift, and is believed to have led to the pandemics of the 20th century.
