The title is derived from Dashiell Hammett's novel Red Harvest. Although there is no single official definition, being blood simple, or getting the fever, as Hammett later refers to it, is being surrounded by murder until murder becomes in your own mind a viable solution to practically any problem. Life becomes cheap, killing begets more killing.
"What have you been doing?" she demanded as we carried our drinks into the dining room. "You look ghastly."

I put my glass on the table, sat down facing it, and complained:

"This damned burg's getting me. If I don't get away soon I'll be going blood simple like the natives. There's been what? A dozen and a half murders since I've been here. Donald Willsson; Ike Bush; the four wops and the dick at Cedar Hill; Jerry; Lew Yard; Dutch Jake, Blackie Whalen and Put Collings at the Silver Arrow; Big Nick, the copper I potted; the blond kid Whisper dropped here; Yakima Shorty, old Elihu's prowler; and now Noonan. That's sixteen of them in less than a week, and more coming up."

She frowned at me and said sharply:

"Don't look like that."

I laughed and went on:

"I've arranged a killing or two in my time, when they were necessary. But this is the first time I've ever got the fever. It's this damned burg. You can't go straight here. I got myself tangled at the beginning. When old Elihu ran out on me there was nothing I could do but try to set the boys against each other. I had to swing the job the best way I could. How could I help it if the best way was bound to lead to a lot of killing? The job couldn't be handled any other way without Elihu's backing."

-- Dashiell Hammett, Red Harvest, 1929