Ike Davis is being treated for Valley Fever. He hasn't received an official diagnosis and the blood test results from a test administered 11 days ago aren't in yet, but the Mets are treating Davis as if he has Valley Fever.
This is the type of illness that can interfere with a season or even end a career, so this is actually pretty serious. Conor Jackson contracted Valley Fever a few years ago and had to miss most of the 2009 season. Hopefully, Ike has better luck than Jackson.
For more on this bummer of a story, here's the article from MLB.com.