From 2006-2008 he hit .290/. 370 OBP/ .450 Slg and .820 OPS. But since 2009 he went from being on the cusp of being the star in Arizona to a spare part. He's bounced from Arizona, to Oakland to Boston in which he hasn't hit over .249 and bounces from 1B / LF /DH and even some 3B. After two dismal seasons last year was the best of them appearing in 114 games while hitting .244.
So what happened to Jackson, Valley Fever...
Jackson was diagnosed with Valley Fever in May and has been on the disabled list since May 12.
Valley Fever is actually Coccidioidomycosis, but got its more well-known name, according to azhealthinfo.org, because it was discovered in California's San Joaquin Valley. It occurs when a pair of spores, which typically reside in soil, are inhaled. It is not an uncommon ailment in Arizona as it's the fourth most common disease reported to the Arizona Department of Health Services. source MLB.com
"I haven't lifted a weight, I haven't run, in three weeks pretty much," he said. "We haven't even talked about timetable. One doctor told me, 'You're going to be fatigued for the rest of the year.' The infectious-disease guy said everybody reacts differently, so I don't know what to expect." source AZ central
In 2008, Jackson hit .300, scored 87 runs and drove in 75, but the following spring, he fell ill with what he thought was the flu. He felt run down, his joints ached, but he tried to play through it. He'd nap after batting practice.
Jackson rapidly began to lose weight, 35 pounds in all. After a month, he'd get home from games and immediately go to bed and he wouldn't wake up until 30 minutes before the team stretch the next afternoon. When he slept right though team stretch, he finally told the trainer, "I think I need to see a doctor."
Valley fever was confirmed via a blood test, and Jackson didn't play again that season. He tried to come back in July, but he would be so fatigued after playing an inning of minor-league ball, he could barely stand up again.
"People didn't recognize me," he said. "I was so pale. For two months, it was a struggle just to get up to go to the bathroom. " source SF Gate
Granted Jackson was an extreme case, but as stated in the AZ central article everybody responds differently. Some have described Valley Fever as mono on steroids and in the MLB article Conor goes on to describe his fatigue as feeling after five innings like he played 20 with a weighted back-pack on.
There are many who don't experience any symptoms, others have flu like symptoms and then the reality is that there are those who experience an extreme case like Jackson. To me it sounds similar to Lyme's disease in the northeast, swap ticks for spores and put it in Arizona.
I wonder out loud if this is why his ankle was so slow to heal or are they two separate incidents...
Davis, reluctant to speak publicly about the illness, did so Saturday. "I have to keep my immune system strong," he said. "No one has told me for sure what it is, and I haven't asked. But they're pretty positive it is [Valley Fever]. I'm trying to be positive about it. I'll just be careful the way they tell me to be careful."source MLB.com