The non use of the proposition “by”

Please consider the following question below:

“Did you ever stop to consider all the germs you pick up dragging a stupid blanket around?”

Is this sentence correct? Shouldn’t the sentence go like this:

“Did you ever stop to consider all the germs you pick up BY dragging a stupid blanket around?”

Shouldn’t the preposition “by” come right before dragging and if it needn’t be so, why?