Heartworms are roundworms that spread to your animals via mosquitoes. They are found in all 50 states and can live anywhere between 5 and 7 years inside your pet. The best way to protect your dogs and cats against heartworms is to provide them with year-round prevention. Symptoms are not usually evident until it is too late and heartworms work fast. They do not go away without treatment and though they can be tested for in dogs, they can not be tested for in cats so prevention is much more important.
More information on heartworms can be found here.