The Hidden - The Other Ones, #1

They told him it was just a parasite... Jack Fuller's life took a swan dive after he cheated on his wife two years ago. His most recent spate of "bad luck" is a list of symptoms that don't seem to fit any particular disease profile. Internet searches and medical hip shots are as close as he can get to an actual diagnosis. Things get worse when a mysterious blood deposit with no apparent source is found in the middle of his bed.
Then there is the intense hunger to contend with, the repeating erotic nightmares and the panic attacks. When the cause of the illness finally manifests, it is wilder than Jack could ever have imagined. The illness has a name. Its name is Jerry.