Heartworms are parasitic worms that primarily affect dogs, but can also infect cats and other animals. They are transmitted through the bite of an infected mosquito, which introduces the larvae into the animal's bloodstream. Once inside, the larvae mature into adult worms that can grow up to a foot long and reside in the heart and lungs, causing serious health issues. If left untreated, heartworm disease can lead to severe complications, including heart failure and death. Regular veterinary check-ups and preventive medications are essential for pet owners to protect their animals from heartworms and ensure their overall health.