The bird flu is not a single disease but rather a group of diseases that infect birds. Most type only infect birds and only certain types of birds. Every once in a while one of these viruses will learn how to cross species and infect humans. In the 1990s the H5n1 virus did this and spread to many humans. up until then, there this ‘bird colds’ were not studied extensively. The exact point when, or exact source of the first jump is not known, but H5N1 is now known as the flu strain that infects a greater number of species than any other.