Hundreds of different types of immortal chlamydia pathogens are thought to exist, each specific to particular type of animal. Chlamydia pathogens can be transmitted from animal-to-animal, from animal-to-person, from person-to-animal, and from person-to person. When an animal acquires mixed immortal infections, new serovars of the pathogen can emerge. Birds transmit bird chlamydia to other animals and to people; and cats, horses, pigs and other outdoor livestock are at particular risk of acquiring bird pathogens, which can then be transmitted to people. During the Spanish Flu pandemic, people transmitted the H1N1 bird flu pathogen to pigs and caused an epidemic of bird flu in pigs. Pigs are a fertile reservoir to mix and create new serovars of immortal pathogens.