Heartworms, called Dirofilaria immitis, are very thin, thread-worms. You won’t see these worms yourself though, as they live within the right side of the heart, and the main artery leading from the heart to the lungs. Heartworms can reach 30cm long, which makes even a few worms incredibly dangerous for a ferret.
Heartworms are nasty, and thankfully are not found in the UK. They are transmitted by mosquitos, and are therefore found in warmer countries, such as those in southern Europe.
Heartworms reproduce by producing tiny offspring, called microfilaria. These are ingested by feeding mosquitos, mature, and are passed on to other animals when the mosquito feeds again.