Rats and Lice
Lice
Lice was the cause of trench fever in WW1. Trench fever was a very nasty and painful disease that began with lots of pain and then a high fever. Even though this wasn't usually fatal, trench fever would weaken soldiers, which required them to have a recovery time of two to three months. Doctors did not find out that lice was the cause of trench fever until the year 1918. Lice sucked the blood of someone infected by trench fever then spread the fever to a successive host.
Rats
Rats first started to become a problem when soldiers would try to sleep at night, they would creep around all of the soldiers and they were not small rats, they were the size of small cats. They would go through discarded food cans. At one point soldiers started to use their bayonets to stap the mice as sport, but were not aloud to shoot at them because they wanted to save ammunition. The rats loved the trenches because of the water, food and shelter.