Is humour purely subjective or are there "rules" about what's funny and what isn't?

Are there “jokes” that are never funny? Subject that should never be joked about?
Or is it about what the listener and the author find funny due to their own beliefs and opinions?

What would make something funny?

Is there a formula to make a good joke? Should there be a “set of rules” that would define what is funny and what isn’t?