Rats are dangerous to man because they carry diseases, damage buildings and wires and create unsanitary living conditions. When an area has a small population of rats, the rats can keep their presence a secret from potential predators. Someone can have rats in his home and not be aware of it.
Rats can transmit diseases through their urine, saliva and fleas. Leptospirosis, an illness that can cause kidney and liver damage, is transmitted through rat urine. Lymphocytic choriomeningitis, a viral infection that affects that brain and spinal cord, can be transmitted via rat saliva and urine. The bubonic plague was spread by a rat's fleas when the fleas bit humans.
Rats love to chew, especially on wood and plastic, causing damage to homes and businesses. Dirt and grime are often tracked by rats throughout a home.