A colorized electron microscope image captures delicate chains of streptococcus in a laboratory sample. Though some strep infections can be deadly, many strains are harmless—among the thousands of benign beings that make their home in our bodies.
So is that breath you just took. When we inhale, our nostrils capture millions of invisible particles: dust, pollen, sea spray, volcanic ash, plant spores. These specks in turn host a teeming community of bacteria and viruses. A few types may trigger allergies or asthma. Far more rare are inhaled pathogens that are themselves the agents of diseases, such as SARS, tuberculosis, and influenza.
Over the past 15 years I’ve spent a lot of time poking cotton swabs up human noses, pig snouts, bird beaks, and primate proboscises, looking for signs of such agents before they cause deadly pandemics. As a result, I’ve come to think of air as the medium for the next pandemic rather than the means to sustain life. But breathe easy: Most of the microbes in the air do us little or no harm, and some almost certainly do us good. The truth is, we still understand precious little about them.
We have known about bacteria, which make up much of the mass of life on Earth, only since Antoni van Leeuwenhoek began training his microscopes on samples of pond water and saliva some 350 years ago. Viruses—much smaller than bacteria but far more numerous than all other life-forms combined—were discovered not much more than a century ago, when people were already driving around in automobiles. And it is only in the past few decades that we have come to realize how ubiquitous microbes are, flourishing from the tops of clouds to miles below the Earth’s surface. We’ve just begun to understand how vital they are to our health and to the health of the Earth. We pride ourselves on having explored nearly every corner of this planet, but behind our world is a shadow world of microbes—and they are often calling the shots.