A: False. The heartworm is an actual worm that can grow to be twelve inches or longer. It lives primarily in the pulmonary blood vessels (vessels that carry blood to the lungs to pick up oxygen), but in bad cases, the worms can even infest the heart itself.
These worms cause high blood pressure, difficulty in breathing, and eventually death due to heart failure. Heartworms are transmitted between animals by mosquitoes. When the mosquito feeds on the blood of an animal that is infected with heartworm, an immature form of the worm, called microfilaria, enters the mosquito. This immature form is not capable of infecting other animals immediately; it requires an incubation period at a constant warm temperature over several weeks inside the body of the mosquito in order to reach the infective stage. Because of this, the danger of infection for pets begins in the spring after a few weeks of warm weather.
Source: College of Veterinary Medicine, University of Illinois at Urbana-Champagne