In nearly every case, a screaming headache would hit full force within hours, and he'd miss another night's sleep.

Over the next several months, Diana says, her husband — a man she says chafes at being indoors too long — had to shut himself inside the house for days on end, and she was scared by the transformation that caused in him.

His eyes often looked vacant and had bags, his mood was always low, and he was more lethargic than she had ever seen him.

"[Brian] is a godly, happy man," says Diana, a Fort Lauderdale native. "And I've seen him go through every injury under the moon, and still he still wants to play. I have to tell you, there were times I had to call [the doctor] and ask what was going on because I was afraid to leave him in the house by himself."

By mid-November, the time by which Roberts usually starts his off-season training regimen, it was clear something was very wrong. It wasn't for another six months that either Roberts really understood what.

Not again

In the first inning of a game on May 16, 2011, the Orioles' 40th of the season, Roberts stepped into the batter's box at Fenway Park.

He lined the first pitch he saw off the ribs of Red Sox hurler Daisuke Matsuzaka. Dustin Pedroia, the second baseman, charged the deflected ball, then threw.

And Roberts did what he often does on a close play at first: He went into a headfirst slide.

His chest hit the ground, his helmet slammed over his face and his head jerked back.

"It was similar to what happens when you're hit from behind in a car — a whiplash effect," Roberts says.

Over the course of that off-season, his general program of rest had, indeed, helped the player feel better, to the point that the headaches were few and far between, and when they did occur, he attributed them to being sick. At spring training, his progress sufficiently impressed team doctors that they cleared him to start the new season.

Within seconds of hitting the ground, though, his head was throbbing with another concussion.

"Please, not this again," Roberts thought.

Concussions are mysterious by nature, and many who get them — including athletes — sometimes hope they'll just go away. (Roberts, it should be noted, stayed in that game, going 0-for-4 the rest of the way. He didn't report his symptoms until the following day, after experiencing more fogginess while playing cards during a rainout.)

Why, for instance, should a person sustain a concussion when conking his helmet with a bat — a blow that few who saw it on TV apparently considered especially violent? How can concussion occur when there's no direct blow to the head at all?

"The general public still doesn't understand how complex and variable this injury is," says Collins, the Pittsburgh specialist. "No two concussions are alike. Neither are the ways in which they affect people."

Among other things, Collins says, research shows that some human beings — athletes or not — are, through no fault of their own, more susceptible to concussion symptoms than others.

A person is more vulnerable to concussion, he adds, after experiencing an earlier one — if and only if the first one never had a chance to heal fully.

And a direct blow isn't always necessary.