Randy Travis -- Family and Friends 'Frantic' About Singer's Condition

The people closest to Randy Travis are extremely worried about the singer's condition ... with sources telling TMZ his health had declined pretty rapidly in recent days after coming down with what he initially thought was a bad cold.

Sources familiar with the situation tell TMZ ... just 3 weeks ago, 54-year-old Travis completed his court-ordered rehab stemming from a 2012 drunk driving arrest ... and was finally back on the road.

We're told ... once Travis began touring, he came down with what he thought was a cold ... but turned out to be pneumonia.

Our sources say Travis' health deteriorated quickly ... and Sunday he was airlifted to a hospital, where he was treated for complications stemming from viral cardiomyopathy.

One medical expert tells TMZ viral cardiomyopathy is not necessarily a death sentence ... and some patients can survive, depending on the level of permanent damage to the heart muscle.

Travis' camp hasn't released the particulars about his medical situation -- only to say he's in critical condition.

Still, we're told Randy's friends and family are "frantic."