Major modern diseases such as Ebola and salmonellosis are zoonoses. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved into a separate human-only disease.[5][6][7] Human infection with animal influenza viruses is rare, as they do not transmit easily to or among humans.[8] However, avian and swine influenza viruses in particular possess high zoonotic potential,[9] and these occasionally recombine with human strains of the flu and can cause pandemics such as the 2009 swine flu.[10] Zoonoses can be caused by a range of disease pathogens such as emergent viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic.[11] Most human diseases originated in non-humans; however, only diseases that routinely involve non-human to human transmission, such as rabies, are considered direct zoonoses.[12]
Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from non-humans to humans through media such as air (influenza) or bites and saliva (rabies).[13] In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting sick. The term is from Ancient Greek: ζῷον zoon "animal" and νόσος nosos "sickness".
Host genetics plays an important role in determining which non-human viruses will be able to make copies of themselves in the human body. Dangerous non-human viruses are those that require few mutations to begin replicating themselves in human cells. These viruses are dangerous since the required combinations of mutations might randomly arise in the natural reservoir.[14]