How can a disease be transmissible animal-to-human but not human-to-human?
I have heard some debate about whether or not the Wuhan virus can be transmitted human-to-human, but this doesn't make sense to me. Why wouldn't it be able to?


A virus has to be able to physically get out of one person and then into another in order to infect them.  If it can't do that, it can't spread between people.  Just because you can get a virus by (for example) eating an infected animal doesn't mean you'll be able to spread it to other people, at least without them eating you.


  Are there diseases that can only be transmitted from animals, not humans?


Yes, and this is very common.  Rabies for instance can be transmitted from hundreds or maybe even thousands of animals to humans, but is virtually incapable of moving between people because it infects the nervous system which severely limits the ability of the virus to get into another host.