Fast vs quickly

[micaelo]

In the following fill-in-the-gap exercise:
Sometimes an allergy can disappear as ... as it arrived, without any treatment.
Would it be allright to use "fast"? The answer key only mentions "quickly/suddently". Thank you.

[Niall]

Sadly, in this case fast cannot be used. The word you are looking for describes disappear, which is a verb. This means, you have to use an adverb.
Fast is a verb, so it cannot be used here.
As the answer key says, quickly is probably the best answer to use here, as it is an adverb.

[engee30]

...
Fast is a verb, so it cannot be used here.
...

Hmm, I am absolutely certain that the asker is aware of the fact that fast is also a verb. The thing is he wanted to knw why fast (the adverb) couldn't be used as one of the possible choices. That's the case, I think, Niall.

[Niall]

Ah. I had neglected to remember the usage of "fast" as an adverb.

In fact, I would change my mind completely from my first answer. And say that "fast" can be used here.

However most people would be more inclined to use quickly