Long after the reversal of that river, the mothers I meet on the beaches of Lake Michigan do not worry much over filth. Most of us believe that dirt is good for our kids, but some of us are wary of the grass in the parks, which may or may not have been treated with toxic chemicals. The idea that “toxins,” rather than filth or germs, are the root cause of most maladies is a popular theory of disease among people like me. The toxins that concern us range from particle residue to high-fructose corn syrup, and particularly suspect substances include the bisphenol A lining our tin cans, the phthalates in our shampoos, and the chlorinated Tris in our couches and mattresses.

I already practiced some intuitive toxicology before my pregnancy, but I became thoroughly immersed in it after my son was born. As long as a child takes only breast milk, I discovered, one can enjoy the illusion of a closed system, a body that is not yet in dialogue with the impurities of farm and factory. Caught up in the romance of the untainted body, I remember feeling agony when my son drank water for the first time. “Unclean! Unclean!” my mind screamed.

“He was too pure,” a Baltimore mother said of her son, who developed leukemia as an infant. His mother blamed the pollutants in vaccines for his illness, and herself for allowing him to be vaccinated. Fears that formaldehyde from vaccines may cause cancer are similar to fears of mercury and aluminum, in that they coalesce around miniscule amounts of the substance in question, amounts considerably smaller than amounts from other common sources of exposure to the same substance. Formaldehyde is in automobile exhaust and cigarette smoke, as well as paper bags and paper towels, and it is released by gas stoves and open fireplaces. Many vaccines contain traces of the formaldehyde used to inactivate viruses, and this can be alarming to those of us who associate formaldehyde with dead frogs in glass jars. Large concentrations are indeed toxic, but formaldehyde is a product of our bodies, essential to our metabolism, and the amount of formaldehyde already circulating in our systems is considerably greater than the amount we receive through vaccination.

As for mercury, a child will almost certainly get more mercury exposure from her immediate environment than from vaccination. This is true, too, of the aluminum that is often used as an adjuvant in vaccines to intensify the immune response. Aluminum is in a lot of things, including fruits and cereals as well as, again, breast milk. Our breast milk, it turns out, is as polluted as our environment at large. Laboratory analysis of breast milk has detected paint thinners, dry-cleaning fluids, flame retardants, pesticides, and rocket fuel. “Most of these chemicals are found in microscopic amounts,” the journalist Florence Williams notes, “but if human milk were sold at the local Piggly Wiggly, some stock would exceed federal food-safety levels for DDT residues and PCBs.”