Then shouldn't we just eat diseases to build immunity?

We do, all the time, by simple manner of eating foods that are exposed to air.

But if you’re asking if we should try to cram more than usual in there, I couldn’t tell you. I’m not an immunologist, so I don’t know how easy it is or is not to overload it, nor if it even works for all types of contagious infection agents. Some are probably highly resistant to gastric environments.