Mosquitoes transmit a variety of viruses and parasites to humans and animals, including West Nile encephalitis, St. Louis encephalitis and Western equine encephalomyelitis virus, canine heartworm disease and malaria. Mosquitoes and Disease Mosquitoes can be annoying, but the main hazard is that they are vectors (transmitters) of disease. Nearly 70% of diseases that pose a threat to global public health are transmitted from animals to humans