Like snakes, viruses have a reputation as malevolent, poisonous, and deadly. In fact, most snakes are harmless, and dangerous viruses are rare. In order to inflict serious harm, a virus has to clear several biological hurdles. First, it has to remain unrecognized by the human immune system – to evade any protective antibodies. The virus would also need to make human sick. (Most do not.) Finally, it would have to spread efficiently – for example, through coughing, sneezing, or shaking hands. Many viruses fulfill one of these criteria; some fulfill two; far fewer meet all three.
– Michael Specter, “The Doomsday Strain,” in The New Yorker