Hosts was the scariest book I've read in the RMJ series.
The one scene that really frightened me wasn't a blood bath, unnamed menace lurking in the dark, or a shoot out. The one that frightened me was the alternative world dream that Jack had. Seeing the world changed to the point where people were willing to bleed themselves out in order to spread a thinking disease was God-awful.
In all the other RMJ books I've read, Jack always had a last fallback plan that he could utilize. The chips were down, but he still had the holdout Semmerling in his pocket, or a pocket lighter or Something.
The inevitability of the disease in the dream world, and the helplessness that Jack felt as everyone around him died, was more horrifying than any savage monster, Otherness entity, or hovering Lars Mao anti-Christ wannbe could ever generate.
Hosts was scary because I saw a way that Jack could fail.