Andrew Miller sends this in: “This sounds like a reference to some phrase. ‘Friends with benefits’ is the only similar thing I can think of and would be a stretch at best. Also not sure why one dog has opposable thumbs and a smartphone. Is the joke just “fleas are contagious?” That would be less of a joke and more of a useful tip for a child that just discovered them.

Since I have space in the comment box and it’s come to mind, I’ll recite De Morgan’s Siphonaptera:

Great fleas have little fleas upon their backs to bite ’em,
And little fleas have lesser fleas, and so ad infinitum.
And the great fleas themselves, in turn, have greater fleas to go on;
While these again have greater still, and greater still, and so on.”