No, there are other explanations. This is from Wikipedia so take it with a grain of salt:

"The mechanism of its antiparkinsonic effect is not fully understood, but it appears to be releasing dopamine from the nerve endings of the brain cells, together with stimulation of norepinephrine response. Furthermore, it appears to be a weak NMDA receptor antagonist and an anticholinergic.

The antiviral mechanism seems to be unrelated. The drug interferes with a viral protein, M2 (an ion channel), which is needed for the viral particle to become "uncoated" once it is taken inside the cell by endocytosis."

If I remember right, the cough syrup ingredient dextromethorphine is also an NMDA antagonist and of course acetylcholine being limited would purportedly help us. So there are several answers hinted at. You mentioned that you had dropped it for awhile. I wonder if it regained its effectiveness and would it be a good strategy to keep it for those "special" ocassions?

But... this doesn't rule out a virus of course. The German scientist Braak has mapped out a pattern of Lewy bodies that sure sounds like the pattern followed by the virus that produces polio.