According to the statement released by the Lester family, Jon suffers from Anaplastic Large Cell Lymphoma. This is a form of Non-Hodgkin lymphoma and thus a form of cancer. I won't try to summarize the disease, the prognosis or the treatment. My medical knowledge is far too limited for me to be able to do that in a credible way. What I can do is provide you with some links to different web sites so that you can easily find information on the condition if you want to read up on it. Here are a bunch.
I apologize profusely for resorting to cliché, but this is the type of news that puts things in perspective. Losing streaks, fielding a AAA lineup and a useless bullpen all suddenly seem a lot less important. Again, I'm no medical expert, but I know enough about cancer to have tremendous respect for it in all its ugly shapes and forms.
I'm not by any means a religious man, but I'll nevertheless be saying a prayer for Jon and his family.
God bless, and good luck. Red Sox Nation will keep you in our hearts.