SAT Reading Inference practice question

The ailment, colloquially known as the 'whispering disease,' has perplexed medical professionals for decades. Its symptoms, including a gradual loss of motor control and a distinctive, barely audible vocal tremor, are highly variable in onset and progression. Despite extensive research, a definitive cause remains elusive, hindering the development of targeted treatments.

What does the passage suggest about the nature of the 'whispering disease'?

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