Dend virus ?
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Even with a pic, there's not going to be any way to positively id a virus infection from photographs; if you really wanted to know, you'd have to send a sample off to a lab for testing. Spotting can be caused by bacteria, insects, fungus, virus, toxic spray residue--all sorts of things, so a plant that's been harmed in some way will very often react by developing a necrotic area (the spot), an area around the spot that turns yellow, and eventually, a loss of the leaf. While certain viruses leave characteristic-looking spots (concentric rings, for instance), others don't, so brown spotting could literally be anything.
If all of the leaves are suddenly yellow after you've put the plant outside, the plant may be getting too much light before having been acclimated to those light levels. If there's any new growth, has that been affected too? Many times, dendrobiums will shed leaves from an old cane and even yellow and drop the entire cane in preparation for a new growth spurt or blooming. You might have several things going on at once which aren't virus-related at all. I have never heard of virus killing an orchid in a matter of days--it's usually a slow, debilitating process that takes months, sometimes years. If the plant looked healthy when you got it 2 weeks ago, developed spots only last week, and now looks like it's on death's door, I can pretty much guarantee that virus isn't the culprit.
Make sure you haven't overwatered, that the roots and medium are in good shape. Check the undersides of the leaves for insects or insect damage: this is a bad time of year for those, so you might try spraying with insecticide too. If you watered the plant outside and the sun hit it before the leaves had a chance to dry, that will also create "rings" of yellow where the water drops acted like magnifying lenses and burned. An infection may have set in after that initial damage. If the problem is fungal and an infection has taken hold, one application of fungicide may not eliminate it, so you'll have to repeat.
Nobody wants virused plants, but detecting viruses when you're not a virology expert is a little like going to your first Psych. class and reading your first textbook: all of a sudden, you "realize" you have all the symptoms of the worst psychoses out there, and you secretly wonder why you're not in the funny farm. Different diseases can manifest in similar ways (spotting, yellowing, leaf loss) and it's very easy to start thinking "virus" when it's really something else, something treatable that won't show up again with good culture.
In any case, based on your description and the time frame, I seriously doubt your Dendrobium has a virus problem, so check those other things first....
