It's a delicate subject, and people's lives are at risk, so I'll state right here, up top, that I do not intend to make light of this public health concern. I share the sympathies that we all have for individuals afflicted by the swine flu. (I've experienced a delirium-packed, 10-day version of the usual seasonal flu, and I wouldn't wish this illness on my worst enemy.)

That said, the reactions of the investing community already look ridiculous: "Markets Down on Swine Flu" read the headlines. Other writers will try to convince you to pile into vaccine names like GlaxoSmithKline (NYSE:GSK), or companies like Netflix (NASDAQ:NFLX), for which a simplistic, "stay-at-home" argument can be made. This is simply rank trend speculation in reverse.

How to really profit
If you really want to find opportunities relating to the swine flu story, I suggest you do the opposite of what most people are advocating. For instance, consider inverting one particularly brazen and short-sighted call that was reported by Bloomberg this morning: UBS downgrades Mexican stocks from "top pick" to "underweight" because of the swine flu.

Really? An entire country's strongest businesses will be permanently impaired because of this health crisis? Would you write off entire segments of the U.S. economy if the illness got worse here? Would you sell Procter & Gamble (NYSE:PG)? Ditch Home Depot (NYSE:HD)?

Sure, the Mexican economy is generally more fragile than ours, but most of the big-name firms trading on our exchanges are anything but weak. Beverage and minimart king FEMSA will likely sell fewer soft drinks and beers over the coming weeks. Will Gruma sell fewer tortillas, Industrias Bachoco fewer chicken chunks? Probably.

Will this matter for the long term?

Very unlikely
If you are investing in strong names for the long term -- and that's how you should be investing -- these are the times when you should be more interested in buying stocks, not less. Flu epidemics are terrible, but they're also normal. So are economic cycles and (in Mexico) the occasional currency panic.

Buying good companies when the headline news is bad is the hardest thing to do (psychologically), but it's the simplest way to buy low. And buying low makes it a lot easier to sell high.

That's the takeaway from the two wealthiest investors in the world -- Warren Buffett and Carlos Slim, who made their fortunes buying companies with competitive advantages on the cheap, often during times of uncertainty. Despite recessions, oil shocks, currency convulsions, SARS, and bird flu, Berkshire Hathaway (NYSE:BRK-A) (NYSE:BRK-B), Telmex, and America Movil (NYSE:AMX) have made them very wealthy.

We've recently revamped Motley Fool Hidden Gems, putting real money into small-cap stocks, to enable us to take advantage of exactly this kind of short-term market craziness. At times like this, we're more interested in our favorite Mexican stocks: Grupo Aeroportuario del Sur and Grupo Aeroportuario del Pacifico. As monopoly airport operators with high fixed costs, both would see turbulence due to a temporary dip in air travel (one they're already getting thanks to the economy).

But in the long term, monopolies like these thrive and enrich shareholders. Ditto the major players I mentioned further up. So unless you think Mexico is forever on the wane, it's time to look at buying these stocks, not selling them.

If you want to read our take on these companies, and to take part in the deliberations with our world-class investment community, a free trial is on the house. Click here to get started.

Seth Jayson is co-advisor at Motley Fool Hidden Gems. He owns shares of Grupo Aeroportuario del Sur, FEMSA, and Berkshire Hathaway. Grupo Aeroportuario del Pacifico and Grupo Aeroportuario del Sur are Hidden Gems recommendations. Berkshire Hathaway and Netflix are Motley Fool Stock Advisor selections. Berkshire Hathaway and The Home Depot are Motley Fool Inside Value selections. Procter & Gamble is a Motley Fool Income Investor recommendation. America Movil and FEMSA are Global Gains picks. The Fool owns shares of Procter & Gamble and Berkshire Hathaway. The Fool has a disclosure policy.