An infectious theory is one that’s only worth is the proof that it can be read. Obviously, it proves this via the process of you reading it. However, the theory also proves this fact via some Rube-Goldbergian method of explaining the universe, which simplifies itself further and further until it states “… thus, this can be read,” or something of that manner. Basically it shows a method of induction that presents an end result of apparent reduction (deduction). That previous statement being proven, of course, via you being able to understand the concept of an infectious theory, thus allowing for one to be read.”
(The above paragraph is an example of an infectious theory)
An infectious theory could be thought of as "pork barrel tautology", as it attempts to sneak some controversial or unprovable idea into a simple, obvious statement. It is not a classically logical way of proving something, as the "proof" for the readibility of an infectious theory may or may not adhere to Occam's Razor - but nevertheless provides instant food-for-thought for basically any crazy position someone might have about the nature of reality; thus it is "infectious". It is generally considered a good idea to make an infectious theory as verbose as possible.
Infectious Theory, then, is not actually a theory in itself - but a way of organizing a theory for which one has no way of proving. The infectious theory gives an easy (albeit incorrect) way out, as long as someone can show how their theory explains the fact that someone can read it. This is far less difficult then you might think.
The term “infectious theory” may also relate historically to the “theory” that germs exist.