The reigning Masters champion recently said that doctors are treating him for psoriatic arthritis which first flared up a week prior to the US Open.
"I have this thing called psoriatic arthritis where your immune system attacks your joints and tendons and so forth," Mickelson told reporters at the Whistling Straits golf course where he is set to compete in the 92nd PGA Championship.
Condition is treatable
"It is very treatable and the medicine I've been taking has been very helpful. I feel 90%."
This is just the latest in a series of misfortunes to hit the Mickelson family.
He has taken a number of breaks from the PGA Tour over the past year to help his wife and mother who are both battling breast cancer.
If he goes ahead as expected, and competes this week, the world number two could surpass Tiger Woods for the top spot with a strong performance at the final major championship of the 2010 season.
"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%, 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."
The left-handed American had a chance to overtake Woods last week, but stumbled to a final round 78 at the World Golf Championships Bridgestone Invitational.
Remarkably Mickelson also played well enough at this year's US Open to give him a shot to win despite the initial onset of the disease.
"I am about back to 80% of the weight I was at before so things look good," said Mickelson who is the odds on favourite among the Vegas bookies to win in Wisconsin. "I am not sure where I will be on Thursday but hopefully I will be ready."
Mickelson, who also said he has become a vegetarian because of this, first visited his physician, but then decided to get another opinion from the doctors at the Mayo Clinic, who are now treating him with weekly shots.
"About eight weeks ago, about five days before the US Open I woke up and had some intense pain in some areas of my body, some joints and tendons and so forth. I couldn't walk," Mickelson said.
"I ended up going and seeing a doctor and I ended up going back to the Mayo Clinic right after the British Open to get a second opinion.
"Everything is fine now. I have been starting treatment and things have been great.
"Long term there shouldn't be any issues."
Won't feel sorry for himself
Asked if he ever wondered why me and my family, Mickelson said he isn't about to feel sorry for himself.
The irony is that he told his wife, Amy, just days before it struck that he felt better than he had in a long time - a rare thing for a professional athlete.
"Stuff happens. It is just one of those things. It happened three or four days before my 40th birthday," he said. "And I was commenting to Amy the week before The Open that I never felt this good. I had no aches and pains. My back felt great. Four days later it is just crazy."
Doctors don't know what causes psoriatic arthritis but treatment can involves anti-inflammatory drugs and steroid injections. In some cases surgery is needed to fix or replaced damaged joints. In Mickelson's case, the drugs and treatment programme are working very well but it could have been worse.
"I have got the best kind that you can possibly have and it is very treatable. It is in remission and I will probably take this drug for about a year and feel a hundred percent. I will stop it and see if it goes into remission. It may never come back and be gone forever."
Mickelson said he kept the diagnosis a secret until Tuesday because he wanted to get more information first about the long-term affects.
"For five or six weeks I was a little unsure of how this was going to affect me long-term and my career.
"Now I feel a lot better about it and I am a lot more at ease to discuss it."
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