'Days of Our Lives' Star Greg Vaughan Hospitalized with Fluid in Lungs

Greg Vaughan's recovering after a scary illness sent him to the hospital ... and, it turns out it's all because he got too high, too fast.

The soap opera star -- best known for playing Eric Brady on nearly 1,000 episodes of "Days of Our Lives" and Lucky Spencer on over 600 episodes of "General Hospital" -- posted a couple photos from earlier this week of him hooked up to an oxygen machine.

Vaughan assured fans he was doing just fine before but had to rush to an urgent care after getting hit with a really bad case of altitude sickness.

GV says he took his kids to Breckenridge, Colorado for a skiing and snowboarding trip, but started feeling tired, congested, and short of breath just a day into what was supposed to be a fun, relaxing vacay.

Greg says he popped some NyQuil and drank a ton of Pedialyte ... but that didn't fix the problem, and he ended up heading down the mountain to seek some medical attention.

While he thought the issue wasn't going to be anything too serious ... Vaughan's lucky he didn't try and tough through this illness -- 'cause he says the oxygen level in his blood was at a paltry 54% and fluid was filling his lungs.

Vaughan says they hooked him up to a couple IVs and told him it was probably best he head for some lower altitude.

Greg's ultimately using his experience as a teaching moment ... telling people the Rockies are no joke -- so be careful on your next ski trip.