Scientists estimate that 90 percent of the cells contained in the human body belong to nonhuman organisms – mostly bacteria, but also a smattering of fungi and other eensy entities. Some 100 trillion microbes nestle in niches from our teeth to our toes.

But what’s setting science on its heels these days is not the boggling numbers of bugs so much as the budding recognition that they are much more than casual hitchhikers capable of causing disease. They may be so essential to well-being that humans couldn’t live without them.

In this emerging view, humans and their microbes – or, as some biologists playfully put it, microbes and their attached humans – have evolved together to form an extraordinarily complex ecosystem.

“We can’t take nutrition properly without bacteria. We can’t fight bad germs without good germs,” he said. “It may turn out that secretions from bacteria affect not only long-term health, but hour-by-hour moods – could a person’s happiness depend on his or her bugs? It’s possible. Our existences are so incredibly intertwined.”

However, in the opinion of some researchers, this strange union may be headed for trouble because of profligate use of antibiotics and antiseptic lifestyles that deter the transfer of vital strains of bacteria that have swarmed in our systems at least since early humans ventured out of Africa.