In the world of health care, PillPack, an online pharmacy, is a pretty small player. Its work force of 1,000 or so people pales in comparison with the 235,000 who work for Walgreens.

But when Amazon announced on Thursday that it was buying PillPack, the deal immediately shook the industry. Shares of Walgreens and Rite Aid tumbled more than 9 percent, while CVS Health dropped 6.6 percent.

That’s because with one move, Amazon answered the question about when — and how — it would grab a piece of the $560 billion prescription drug industry.

It was precisely the sort of deal that the health care industry had feared.

Amazon has been hinting at its interest in selling drugs, but it faced the problem of securing pharmacy licenses in each state. PillPack will help overcome that hurdle, since the start-up is licensed to ship drugs in 50 states — clearing the way for the e-commerce giant to quickly become a major player in the business.