I find it fascinating how this discussion goes, Do we differentiate between "healthy" and "strong" ?

The US economy is, as far as I can tell, tethered in a macroeconomic sense, to the bill of the very expensive land wars it recently fought. The cost to the economy both in terms of government spending and workforce depletion as national guard troops were mobilized for duty in Iraq and Afghanistan.

American worker productivity has remained high, which has kept real wage growth low, as employers leverage they "be thankful you even have a job" meme over the heads of employees who perhaps just scraped by in the great recession or spent months or even years unemployed.

America's largest trading partner, China, is running a partially managed economy with a fiat currency that is priced more on government policy than market realities. That pricing has lead to some unsustainable conditions in the Chinese economy which are being "addressed" by some pretty big moves (currency devaluation is not something you do lightly in the worlds second largest economy).

So the interesting question is how does the world see it? And how will it play out? Everyone has their bets, but and you can read about some of them in Barron's or the Economist or the WSJ.

The current disaster is Chinese. By devaluing their currency they are effectively "taking value" from people who were trading with them relative to the partner's home currency. So lets say someone like Apple contracts to buy 10 million iPhone 6 baseboards, in Rmb at an exchange rate of $100 per board, and now when it comes time to actually take delivery and buy the boards they cost Apple the equivalent of $150 each. Apple needs to pony up an additional half billion dollars for their phones. This then will hit their bottom line in terms of revenue, which means their stock price will go down (they won't be as profitable a company) and so funds holding Apple will lose their value in proportion to their Apple stock. And China has done this by devaluing its currency.

It doesn't change how strong Apple's market presence is, or that they can sell a phone for a ton of money, but it changes the cost/value equation faster than Apple can respond and so there is a disruption in their earnings. That will ripple across a lot of companies.

But is that a 'health' issue for the American economy? Not really. Rather it puts pressure to restructure the costs of the economy into a different place. People still buy iPhones and will for the forseeable future. So the economy is still strong, but if the price of those iPhones doubles their volume will likely fall and so Apple's earnings might be 'weak'.

The stock market is responding to the adjustments in China, internalizing the lack of fiscal oversight in that economy, and pricing it into the value of companies that do a lot of business there. I expect a hell of a correction and some interesting new markets opening up (like India, Vietnam or Thailand if the Thai can get their governance under control) as the cost of doing business in China begins to more accurately reflect the real costs of doing business there.

EDIT: As folks have pointed out the currency hit is reversed, Apple would get its parts for less if they priced them in RMB vs Dollars. Any RMB they were holding in their cash pile would have lost value, so to the extent that their sales in China have not been moved into Euros (we know they aren't repatriated into Dollars for tax reasons) are going to buy less than they did before.