Zoonotic diseases are infectious diseases caused by bacteria, parasites or viruses that can be passed between mammals and people.
The pathogen originates in wildlife (the reservoir host), often bats, that come into close or immediate contact with mammals (the intermediate host). Human interactions with such an intermediate host, through for example petting, handling or management of captive lions, is how the pathogen can jump or spillover from wildlife to humans.
This is more likely to happen when the animal is kept in poor conditions, is not well or under stress, like it often happens in the commercial captive lion breeding industry.
Once this zoonotic jump has occurred, there is a possibility that the infected human can infect other people.