Pest spread disease. They feed on plants, animals and even on humans. Some pests, even suck human blood like bed bugs. Some pest like fleas spread diseases through their sting. My friend's house was infested with cockroaches and fleas. The flea bite was there on his body, hand, legs. The cockroaches drop their droppings everywhere. When this infestation leads to its extreme level. He called Exterminator Greenwich CT for his help for exterminating her house.