Chapter 9 brings up the concept of potentially administering bacteria as a medicine to cure a disease such as obesity or depression. As the chapter details, it isn’t that simple. That is also the case for probiotics that are sold on the market. A probiotic is a bacterium that imparts health benefits when consumed. Many scientists are looking for ways to use bacteria as probiotics to solve disease crises throughout the world. They are also looking at the opposite: eliminating single bacteria. Both routes are plausible, but both come with their own sets of challenges.

Chapter 9 opens by detailing a disease called elephantiasis, which is characterized by a swelling of lymph nodes to extreme sizes, particularly in the groin area. The disease is caused by a nematode that is passed through mosquito bites, which give the nematode larvae a route into the body and into the lymph tissue. A combination of the nematode and a familiar bacterium, Wolbachia, cause the disease. The nematodes need these bacteria to finish their life cycle, and when the worms die, they release the bacteria to trigger an immune response (213). The combination of the two immune responses, one towards the bacteria and one towards the nematode, causes the symptoms of the disease.