That sounds fine. My problem is with the idea that low level characters should be afraid of everything because they have so few hit points. Even a low-level character should not be seriously threatened by a giant rat...that's like your soldier coming back from a tour in war and getting killed by a corgi! I'm not a skilled combatant by any means but I'm fairly certain that it'd take a large number of corgis to seriously injure me unless there were some pretty extreme circumstances.
Spoiler
The disease that rats may be carrying, however, is an entirely different matter.