This is a photomicrograph of an adult tapeworm, Echinococcus granulosus. It is tiny, only 3-6 mm long.
However, it can cause a world of hurt.
E. granulosus, in a larval form, causes cysts to form, generally in the liver and lungs.
How do you get this?
You ingest an echinococcus egg. The eggs are shed in the feces of the definitive host, which is your old pal, the dog. The normal intermediate host is a sheep. Because they eat grass, they have a high likelihood of ingesting an egg in those places where dogs leave their feces. When the sheep are slaughtered, the offal (guts) is tossed on the ground, and the dogs eat it, completing they cycle.
A cyst? What's the big deal?
The big deal is that they can grow quite large. So although they are not malignant masses, they can put pressure on surrounding tissues and cause organ dysfunction. Additionally, the cysts are filled with a fluid, that if it leaks out, causes acute anaphylaxis, possibly fatal.
How do you get this?
The same way that the sheep does: you ingest an echinococcus egg. The eggs are shed in the feces of the definitive host, which is your old pal, the dog. The normal intermediate host is the sheep. Because they eat grass, they have a high likelihood of ingesting an egg in those places where the parasite exists. When the sheep are slaughtered, the offal (guts) is tossed on the ground, and the dogs eat it, and the larvae turn into the adult tapeworms in the intestines of the dog, completing they cycle. People end up ingesting the ieggs because, well, shit and bad hygiene happen.
Here is an adult Taenia solium tapeworm, (also known as the pork tapeworm, the kind you get from eating infested, undercooked pork) being extracted from a patient's intestines via endoscopy (this is not the standard treatment!).
You can see that it's a rather long parasite. This is our stereotypical idea of a tapeworm: a long nasty thing that snakes its way through the intestines. But the size of the adult worm isn't the problem. With both E. granulosus and T. solium, the adults don't cause problems to their natural hosts (the dog and the human, respectively). Problems generally arise when the life stage of a parasite ends up in species that it's not adapted to.
Tapeworms are composed of a head, called a scolex, a neck region, and the strobila, which is a chain of proglottids. The proglottids are the little sections you see making up the bulk of the worm, or if you've been lucky enough to see them in your dog's poop, the little pieces of off-white that look like rice grains wriggling around.
The pork tapeworm has a lengthy chain of proglottids. E. granulosus , the tapeworm of hydatid disease, has only 5 segments: the scolex, the neck and three proglottids. The whole thing is only 3 to 6 mm long.