Possible Clues:

• Vermin seen round sack - cause of disease?
• Plunder overrun by vermin - a source of infection
• Tiny organism that can cause disease
• Tiny organism causing disease
• Are the mice about to steal anything so small?
• Eg, bacterium
• Tiny source of disease
• In absence of cat, players about to steal something much smaller
• Vital bit of fertile soil
• Bug Nick to feed rodents