Viruses are curious things. The tiny tagalongs aren't exactly alive by most definitions, but they're not really inanimate either. They've flourished and diversified for billions of years and perhaps even had a hand—or a squishy protein coating—in helping the first complex cellular life come to be.

While these microbes have a dizzying array of functions and health effects, the structure of a virus is surprisingly simple. Each one consists of genetic material—either DNA or RNA—encapsulated in a protein pocket called a capsid. Some are additionally enveloped in a soft, lipid wrapping. These tiny virus packages are just tens to a few hundreds of nanometers across. This makes them smaller than most bacteria, which can be a small as roughly