A Planet of Viruses - Carl Zimmer [3]
Despite the small size of viruses, scientists discovered ways to dissect them and peer inside. A human cell is stuffed with millions of different molecules that it uses to sense its surroundings, crawl hither and yon, take in food, grow, and decide whether to divide in two or kill itself for the good of its fellow cells. Virologists found that many of the viruses they studied were just protein shells holding a few genes. They discovered that viruses can replicate themselves, despite their paltry genetic instructions, by hijacking other forms of life. They could see viruses inject their genes and proteins into a host cell, which they manipulated into producing new copies of the virus. One virus might go into a cell, and within a day a thousand viruses came out.
Virologists had grasped these fundamental facts by the 1950s. But virology did not come to a halt. For one thing, virologists knew little about the many different ways in which viruses make us sick. They didn’t know why papillomaviruses can cause horns to grow on rabbits and cause hundreds of thousands of cases of cervical cancer each year. They didn’t know what made some viruses deadly and others relatively harmless. They had yet to learn how viruses evade the defenses of their hosts and how they evolve faster than anything else on the planet. In the 1950s they did not know that a virus that would later be named HIV had already spread from chimpanzees into our own species, or that thirty years later it would become one of the greatest killers in history. They could not have dreamed of the vast numbers of viruses that exist on Earth; they could not have guessed that most of the genetic diversity of life can be found in virus genes. They did not know that viruses help produce much of the oxygen we breathe and help control the planet’s thermostat. And they certainly would not have guessed that the human genome is partly composed from thousands of viruses that infected our distant ancestors, or that life as we know it may have gotten its start four billion years ago from viruses.
Now scientists know these things—or, to be more precise, they know of these things. They now recognize that from the Cave of Crystals to the inner world of the human body, this is a planet of viruses. Their understanding is still rough, but it is a start. So let us start as well.
OLD COMPANIONS
The Uncommon Cold
Rhinovirus
Around 3,500 years ago, an Egyptian scholar sat down and wrote the oldest known medical text. Among the diseases he described in the so-called Ebers Papyrus was something called resh. Even with that strange sounding name, its symptoms—a cough and a flowing of mucus from the nose—are immediately familiar to us all. Resh is the common cold.
Some viruses are new to humanity. Other viruses are obscure and exotic. But human rhinoviruses—the chief cause of the common cold, as well as asthma attacks— are old, cosmopolitan companions. It’s been estimated that every human being will spend a year of his or her life lying in bed, sick with colds. The human rhinovirus is, in other words, one of the most successful viruses of all.
Hippocrates, the ancient Greek physician, believed that colds were caused by an imbalance of the humors. Two thousand years later, the physiologist Leonard Hill argued in the 1920s that they were caused by walking outside in the morning, from warm to cold air. The first clue to the true cause of colds came when Walter Kruse, a German microbiologist, had a snuffly assistant blow his nose and mix the mucus into a salt solution. Kruse and his assistant purified the fluid through a filter and then put a few drops into the noses of twelve of their colleagues. Four of them came down with colds. Later, Kruse did the same thing to thirty-six students. Fifteen of them got sick. Kruse compared their outcomes to thirty-five people who didn’t get the drops. Only one of the drop-free