The UN's World Health Organization said on Thursday that it is still investigating the origins of Covid, but the "strongest evidence is still around zoonotic transmission".
And with more than 1,000 monkeypox cases recorded globally over the last month, the Un agency has warned there is a "real" risk the disease could become established in dozens of countries.
While such diseases -- called zoonoses -- have been around for millennia, they have become more common in recent decades due to deforestation, mass livestock cultivation, climate change and other human-induced upheavals of the animal world, experts say.
Other diseases to leap from animals to humans include HIV, Ebola, Zika, SARS, MERS, bird flu and the bubonic plague.
The WHO's emergencies director Michael Ryan said last week that "it's not just in monkeypox" -- the way that humans and animals interact has become "unstable".
"The number of times that these diseases cross into humans is increasing and then our ability to amplify that disease and move it on within our communities is increasing," he said.
Monkeypox did not recently leap over to humans -- the first human case was identified in DR Congo in 1970 and it has since been confined to areas in Central and Western Africa.
Despite its name, "the latest monkeypox outbreak has nothing to do with monkeys," said Olivier Restif, epidemiologist at the University of Cambridge.
While it was first discovered in macaques, "zoonotic transmission is most often from rodents, and outbreaks spread by person-to-person contact," he told AFP.