What, exactly, is a “jump” in your mind. The virus hops on you like a flea? Because what a jump actually constitutes is a mutation or series of mutations which allows the virus to invade to a new host. This inherently represents an increase in fitness with respect to the new environment, pretty much by definition. “Fitness” is all about the fit between the organism and its environment, not some property of how pristine its DNA/RNA!
A fundamental challenge for host-switching viruses that require adaptation to their new hosts is that mutations that optimize the ability of a virus to infect a new host will likely reduce its fitness in the donor host
Cross-Species Virus Transmission and the Emergence of New Epidemic Diseases