The metaphors we use "prime how we think and act" (pg. 12). If many of our metaphors involve the body, then it seems that our bodily existence guides much of our thoughts and actions. Of course, then, we tend to associate "natural" with what is good for us and "unnatural" with what is bad for us. The body is the home we are born with, so even as our lives become more technologically advanced and separated from nature, we still consider the body an aspect of nature. Moreover, many of the "unnatural" components of our modern world, engineered by humans, are in some way harmful. Take pesticides, bombs, and the diseases spread through colonization, for example.
Modern medicine too can be understood as being in opposition to nature: we strive to prolong our lives and perfect our bodies. Though we seek health, it makes sense that, if we associate the natural with the good, we would be suspicious of some aspects of modern medicine. The most notable example of this paranoia involves vaccines. Vaccines, made of live, captured viruses, enter our bodies through needles, violating our natural barrier to the outside world. Perhaps more unnaturally, the viruses in vaccines do not, if successful, "introduce disease or produce illness" (pg. 41). Hence, our fear of vaccinations is... natural. However, we must, like Biss, overcome this fear through thorough research of the benefits of vaccination, which may outweigh the monstrosity of disease.