SHEBOYGAN, Wis. (AP) — Phil Mickelson is being treated for arthritis that surfaced just before the U.S. Open and left him in so much pain he couldn’t walk.

Mickelson revealed Tuesday he has psoriatic arthritis, a condition he said causes the immune system to attack the body’s joints and tendons. Weekly shots of Embrel, which lowers his immune system, have brought the disease under control.

“I’m surprised at how quickly it’s gone away, and how quickly it’s been able to be managed and controlled,” he said. “I feel 100 percent, like I say. But when I’m laying there on the couch and I can’t move, you know, yeah, I had some concerns. But I feel a lot better now.”

He’s also has become a vegetarian — almost as shocking a revelation for the burger connoisseur as the illness itself.

“I know, I know,” he said as reporters chuckled. “As long as I believe that there’s a possibility that it will help me overall, yeah, I’ll continue to do that. If it will somehow keep this in remission or stop it from coming back, yeah, I’ll be able to do it. But I haven’t been put to the real test. The real test is driving by a Five Guys and not stopping.”

The arthritis is the latest health scare for Mickelson’s family. His wife, Amy, and mother are both battling breast cancer; the long-term prognosis for both is good.

Mickelson, who turned 40 in June, said he woke up five days before the U.S. Open with “intense pain” in his tendons and joints that left him unable to move and his joints feeling sprained. Stretching, walking and anti-inflammatories alleviated the pain, and he went ahead and played Pebble Beach, where he shot 66 on Friday to put himself in contention. He wound up tied for fourth.

But the condition got progressively worse during the U.S. Open and a family vacation to Hawaii afterward, spreading to his knees, hips and elbows.

“That’s when I got concerned,” he said. “I certainly had the gamut of thoughts. … I would just lay down and I couldn’t roll over. I was concerned about being able to swing a club and so forth.”

After the British Open, Mickelson made a trip to the Mayo Clinic in Rochester, Minn., where doctors confirmed the diagnosis of psoriatic arthritis. According to the Mayo Clinic website, the condition causes joint pain, stiffness and swelling. While it can be linked to psoriasis, the arthritis can appear without the presence of skin lesions.

There is no cure for the condition, according to the Mayo Clinic website, but the disease is treatable.

“I feel great now and things have been much, much better,” Mickelson said. “I’ll probably take this drug for about a year, and feel 100 percent. I’ll stop it and see if it goes into remission and it may never come back. It may be gone forever.”

“It’s not that it’s cured, but it may never come back,” he added. “Or if it does come back, I’ll start the treatment again and should be able to live a normal life without having any adverse effects. So I’m not very concerned about it.”

Mickelson has been threatening to replace Tiger Woods as world No. 1 since The Players Championship in early May, only to stumble at each opportunity. That’s led many to wonder what’s ailing Lefty, particularly after his lackluster round Sunday at Firestone, where he made seven bogeys and a double-bogey — and just one birdie — on his way to a 78.

While Mickelson said the arthritis didn’t affect his play at Pebble Beach or St. Andrews, where he was never a factor, he only resumed his regular practice routine last week.

“First of all, I don’t want excuses. And second, I don’t want to discuss something when I don’t know what the outcome is going to be,” Mickelson said, referring to why he waited until Tuesday to reveal his condition. “For five or six weeks, I was a little unsure of how this was going to affect me long term, career, what have you. Now that I feel confident it’s not going to affect not only the rest of my career or the rest of my life, but even in the short term it shouldn’t have an effect, I feel a lot better about it and I’m a lot more at ease to discuss it.”

Mickelson, the 2005 PGA champion, is seeking his second Wanamaker Trophy this week at Whistling Straits. He’s also trying to win a second major in one season for the first time. He won his third green jacket at the Masters in April.

“I’m probably not as sharp as I would like to be,” he said. “I didn’t play well at the British, obviously. I didn’t play well last week, on the weekend, but I’m able to work on it. I had a good session with Butch and I believe that the game’s coming around. I’m not sure again where I’ll be on Thursday but hopefully I’ll be ready.”