The majority of emerging infectious diseases around the world is zoonotic. Comprehensive surveys of recognized human pathogens by Woolhouse et al. in 2005 and previously in 2001 found that around three quarters of these diseases have their origins in wild animals as humans are quickly encroaching on their habitat. An ongoing outbreak of the bat-borne Nipah virus in the Indian state of Kerala shows how dangerous these diseases can be, as two of six confirmed cases have led to deaths. The overall mortality from the virus is between 40-74 percent of confirmed cases.
The most recent survey published in the Emerging Infectious Diseases details the reasons for this development, the most important being changes in land use and agricultural practices, changes in human demographics and society as well as the poor health of populations which might encounter zoonotic pathogens. Historically, around half of infectious diseases that can make humans sick have come from animals, while the other half has been human-specific, meaning they only infect the human species.
Most prominent among emerging pathogens are viruses. 77 were counted as part of the study, followed by 54 species of bacteria, 22 of fungi, 14 of protozoa (single-cell, animal-like organisms) and 10 of parasitic worms. A case of the latter occurred in Australia just last month when a parasitic roundworm so far only found in animals was pulled from a woman's brain. The Covid-19 virus, which was first detected in humans in late 2019, is also a zoonotic disease previously found in bats - which are a major carrier of viruses.
