Hail Mary
Let's face facts: humans are filthy creatures. What if there's a communal source of water that's big enough to bathe in, like a lake or a pond? Surely no-one would be stupid enough to drink from that? Dear reader, never overestimate the human potential for doing something stupid. Reward incautious drinkers with a dose of typhoid fever.
Typhoid Fever: Typhoid fever is another bacterial infection, but this time the culprit is caused by Salmonella typhi. This microbe is only carried by humans, but you may wish to broaden that classification to include humanoids and giants of all kinds. In its initial stages body temperature increases, and the victim is subject to headaches, fits of coughing, listlessness and loss of appetite. The disease goes on to infect the intestines, causing diarrhoea and bloody stools.
Infection DC: Ingestion 14
Incubation: 1d10+10 days
Damage: 1d3 Str, 1d3 Con, 1d3 Wis
Additional Effects: The character must make a DC 14 Fortitude save every hour or become sickened.
Typhoid fever is an ideal disease to unleash in cramped conditions. Contaminate the food supply with it. Infect a wandering NPC with the disease and turn him into an angel of death. Test the PCs' intelligence (or lack thereof) with a search for the Typhoid Mary.