As humans encroach on the natural world, more deadly pandemics are likely to follow COVID. Why? Here's everything you need to know:. Nearly all of them start with animal-to-human transmission of a virus or bacteria to which people have no immunity. Animal pathogens are the source of about 60 percent of known infectious diseases and 75 percent of those that appear for the first time in humans. About 250 known diseases have made the leap from animals to humans, but a 2020 United Nations report estimates that as many as 850,000 viruses lurk within the bodies of mammals and birds. Zoonotic diseases have wreaked havoc throughout human history. The bubonic plague, which wiped out up to 60 percent of the population of Europe, Asia, the Middle East, and North Africa in the 14th century, was caused by the Y. pestis bacteria transmitted from rodents via fleas. The 1918 flu pandemic, which killed an estimated 50 million people, is believed to have originated in poultry and wild birds. "Spillover" events appear to be getting more common. HIV and Ebola can be traced back to primates in Africa; the original SARS virus is believed to have jumped from bats to humans by way of a mammal called a civet. COVID is caused by a bat virus, SARS-CoV-2, that recent research has found most likely spilled over from animals sold at a wet market in Wuhan, China, in 2019, although some believe Chinese scientists modified the virus and accidentally let it escape.