Viruses are easily the most abundant life form on Earth, if you accept the proposition that they’re alive. Try multiplying a billion by a billion, then multiplying that by 10 trillion. That — 10 to the 31st power — is the mind-numbing estimate of how many individual viral particles populate the planet. Is a virus a living thing? Maybe. Sometimes. It depends on location. “Outside of a cell, a viral particle is inert,” on its own, it can’t reproduce itself or, for that matter, produce anything at all. It’s the ultimate parasite.
“Know your enemy,” Sun Tzu, the great sage of war, wrote some 2,500 years ago. Today, as COVID-19 spreads around the globe, the greatest army of medical scientists ever assembled is bent on learning all it can, as fast as it can, about SARS-CoV-2, the virus behind the pandemic. However, compared with a lab dish, living people are complicated. The cells in that dish aren’t the same as the cells in living tissues affected by SARS-CoV-2. Plus, the environment surrounding, say, a lung cell in a person’s body is different from the one in a culture dish. And then there’s this thing called “side effects.” You don’t see those in a dish. But you may in a COVID-19 patient.
Viral mutation rates are much higher than bacterial rates, which dwarf those of our sperm and egg cells. RNA viruses, including the coronavirus, mutate even more easily than DNA viruses do: Their polymerases (those genome-copying enzymes mentioned earlier) are typically less precise than those of DNA viruses, and RNA itself is inherently less stable than DNA. So viruses, and particularly RNA viruses, easily develop resistance to our immune system’s attempts to find and foil them.