Designer symbiots were one of the most hyped failures of the biotech industry. Tailored micro and macro-organisms promised automatic, self-regulating augmentation, a tool that could merge with the body's complex systems. Many of these promises even came true. When it came to the market, however, biotech companies were simply unable to overcome the "squick" factor: their products were too gross for consumers. In the end, they were rendered obsolete by medichines and nanotechnology.
The famine worm was designed as a hedge against starvation, able to provide a package of calories when most needed, but the famine worm was also a genetically engineered tapeworm that very few people were willing to host. The worm was first offered to famine and starvation-response charities, where it saw some minor successes. The primary market was in militaries where potential hosts could simply be ordered to eat the eggs, however gross it seemed. Famine worms are no longer sold or supported by any company, but the freeze-dried eggs remain viable for decades and the genome has become public domain. You never know what you might find in some barsoomian supply-cache or old brinker hab.
Acquiring a worm is simple: just swallow a pill containing the egg. Once hatched, the worm anchors itself at the top of the intestinal tract. Like a normal tapeworm, it has a voracious appetite, eating from the digestive system of its host. Once it reaches sufficient size, however, it stops growing, and eats only enough to sustain itself. A famine worm can remain in this low metabolic state for years. If the host (and therefore the worm) ever goes for ~48 hours without eating any food, the worm dies, and begins a genetically programmed phase of rapid disintegration. The disintegrated worm is easily taken up by the intestines, providing a small but timely burst of calories and nutrients.