Prefacing "pitch" with "fever" is a bit unclear, but I believe the key to understanding "fever pitch" rests in phrases like "fevered" and "feverish," all of which describe a condition of overwrought excitement, restless energy, and delirious fury caused by a severe fever. Fever itself is defined as an elevated body temperature, and although most people's bodies maintain their own set point, some diseases (such as malaria) can cause temperatures to rise very high, even though antibodies are produced to fight them off. But only people who have never had a fever know what it feels like when your body is heated up by a virus or bacteria that can no longer be detected by the immune system. They don't feel any different than they would if their blood were boiling hot. But since "pitch" is used here in its ordinary sense of "a high-pitched sound," I'd say this is where the ambiguity lies.