The jump is only the first step
A germ that infects an animal does not automatically cause an outbreak in people. It first has to reach someone—through a bite, contaminated food or water, contact with bodily fluids, or another route—and get past the body’s defenses. This first infection is called a spillover.
What happens next depends on the germ and the kind of contact it can exploit. Some infections remain confined to the person exposed to the animal. Others can spread between people, for example when an infected person releases germs in respiratory droplets or passes them through close contact. If those routes repeatedly connect one person to the next, the infection can continue without further animal exposure.
Why some infections keep spreading
A germ must be able to enter human cells, reproduce there, and reach another person in a usable form. Traits that work well in an animal may not work as well in humans, so many spillovers lead to no further cases. Sometimes changes in the germ help it infect or spread among people, but a single successful infection does not mean that such a change has occurred—or that an outbreak will follow.
The setting matters too. Frequent contact with infected animals can create more chances for spillover, while close, repeated contact among people can give a human-transmissible infection opportunities to move onward. An animal-to-human jump and sustained human spread are separate hurdles.
From one case to a chain
When person-to-person spread is possible, each new infection can become another link in a chain. If those links keep forming, an illness that began in an animal population can establish itself among people. That is why finding a spillover case matters: it marks a crossing, but the crucial question is whether the germ can keep crossing—from one person to the next.
