Perhaps you've heard a joke like this:
Two atoms walk into a bar. One says, "I think I've lost an electron!"; the other, "Are you sure?"; the first, "Yes, I'm positive!"
This is the general structure of most 'academic jokes': they'll play out like a kids joke, except the humour is based around specialised knowledge. They are normally pretty awful. But I still find myself chuckling, and academics do tend to appreciate them and circulate them, groaning as they do so - a similar groan to the "my dog has no nose..." kind of joke.
There are other jokes based around more general observations of your chosen subject, rather than pure facts:
Physics Exam question: Describe the universe in 300 words and give three examples.
But they are all styled quite similarly to the kids' jokes of old: cringeworthy, basic and a few good steps down from pinsharp wit, or a good stand-up act. So why do we (okay, I) chuckle when we've stopped chuckling at kids' jokes?
Some people think that it's to do with feeling the need to chuckle because you enjoy being in the few who properly get the joke. It's a little bit of ego-stroking, an elitist joke, if you will. Perhaps if I told you the chicken crossed the mobius strip to get to the same side you wouldn't know what the hell I was talking about. Or perhaps you do know what I'm talking about, but know that you might be in the minority and so finding the joke funny makes you feel good for being in that group of well knowledged peoples.
For me, personally, it's a slight twist on this. Sometimes I hear a maths joke and I'll laugh. I'll laugh, because I know I'm in the minority of people who will 'get it', but not because it makes me feel better. Rather, I know that someone made this joke and hardly anyone will ever 'get it' and I feel it's my duty to laugh at the poor little joke, preaching to the very small choir; it needs some recognition!
I don't know much about medicine, so if anyone wants to send me some medicine jokes (I'm looking at you,Sanjay) then I'd be happy to not get them.