Some insects are harmful to humans. Locusts can devastate a farmer’s crops in only a few hours. Parasitic insects feed on human blood and can transmit dangerous diseases to its host. Fleas were the cause of the Medieval Black Plague. Lice infest human hair and cause severe itching. Female mosquitoes use human blood to fertilize their eggs. Mosquitoes are known to transmit diseases such as malaria and the West Nile Virus.