We moms want nothing more than to protect our children from harm, and when our babies first arrive, the potential predators can seem endless. Germs are everywhere, so we wash our hands incessantly, shield our strollers with covers, and avoid public places. Most of us only start to breathe easier at six weeks, when our babies have had their first round of immunizations.

But did you know that long before that, you can provide your baby with his first and best defense against the deadly bacteria and viral microorganisms in your environment? One of the most powerful protections is right inside your underwire bra. That’s right: your breasts. By making breastmilk, we moms can construct an immunological fortress around our babies that nothing in science can even begin to approximate.

While we’ve long known that breastfed babies get sick less often than formula-fed babies, scientists have often chalked up this discrepancy to unclean bottles. We knew that breastfed babies lived longer and healthier lives, but not why.  It’s only recently that we’ve begun to understand that breastmilk provides far more than nourishment: it’s the ultimate weapon against bacterial germs, viral infections and childhood illnesses.

Recent archaeological findings point to evidence that mammalian lactation originally evolved as a protective adaptation, with the nutrition playing only a secondary role. This is an amazing scientific discovery—and comforting, too, especially these days, when kids seem to be falling prey to more illnesses and afflictions than ever before.

The milk you give your child is an incredibly potent first vaccine. No wonder babies instinctively push toward the breast immediately after birth; it’s nature’s way of offering them protection.

When babies are born, their guts are sterile and extremely permeable, and the gastrointestinal system is the biggest entryway to bacterial germs and viral infections. Colostrum, the powerful predecessor to mature breastmilk, readies your baby’s gastrointestinal system for its fight against these microbial invaders by swarming the gut with high concentrations of antibodies. Those early infusions of colostrum also physically alter the digestive tract by thickening the intestinal walls and providing an added layer of protection from a bacterial and/or viral attack.

A breastmilk-lined gut doesn’t tolerate lingering microbes for several reasons. First, breastmilk has a low pH level, and bacteria and microbes don’t thrive in an acidic environment. Second, microbes need iron to breed.  Breastmilk contains lactoferrin, a protein that binds to iron, rendering microbes unable to grow and multiply. Hundreds of other ingredients in breastmilk work to destroy or neutralize any microbial invaders that made it through the first two lines of defense. And unlike medicinal antibiotics, which kill everything indiscriminately, breastmilk antibodies can differentiate the good bacteria from the bad.

Formula, by contrast, tends to create a welcoming environment for microbes. Its high iron and pH levels can allow bacteria, viruses and hostile organisms to proliferate. Even one bottle of formula can alter the pH level within the gastrointestinal complex, quickly changing the environment within a baby’s gut from acidic to alkaline. And even if breastfeeding immediately resumes, it can take up to six weeks for the pH level to acclimate, which partially disables the baby’s protective shield in the interim.

While formula companies have spent billions of dollars on research and development, their product just doesn’t come close to the immunological genius of breastmilk. Breastmilk provides infants with the best protection against bacterial germs, viral infections and childhood illness.  It  truly is the perfect first vaccine.