VIRUS

The word “virus” conjures up the fear of death on invisible wings. It raises images of hospital wards crammed with patients dying of Spanish ’flu; poliomyelitis victims in iron lungs; doctors wearing full-body suits against the deadly Ebola virus; or babies with microcephaly that could be linked to Zika virus. 

These are all dreadful human diseases, but they tell only a really small a part of the story.  Viruses are part of the history of life on Earth; precisely what part they play is a mystery that is slowly being unraveled. In this book you will find a more rounded picture of viruses. To be sure, you’ll read about viruses that cause disease, but you will also discover viruses that are actually good for their hosts. So good, in fact, that the hosts couldn’t survive without them. 

Source Wikipedia

Them viruses during this book are chosen to reflect the incredible sort of viruses. Some you will have heard about—others will be new, and strange. Some have played a part in key episodes in the history of science, such as the discovery of the structure of the genetic material, DNA. Others do weird things to the biology of their hosts. 

Viruses cannot live without their hosts, so this book orders viruses by the type of animate thing they infect. Starting with humans, we move to other vertebrate animals and plants. Insects and crustaceans (invertebrate animals) have their own viruses, as do fungi. Even bacteria—some of which are also agents of disease—can be infected with viruses. 

The book includes illustrations to point out off the unique great thing about viruses. Many viruses have, precise geometric structures, made from repeating units of proteins that make up their coats. Viruses of bacteria and archaea have landing gear they use to attach and drill into their hosts like a space probe landing on another planet. Some viruses look like flowers, albeit on a microscopic scale; others have eerily beautiful effects on their hosts.

This introduction contains all the essentials for you to start to understand viruses and how they are studied: the history of virology (the study of) viruses. You’ll discover how viruses interact with their hosts; how they affect their hosts’ interactions with the world around them; and how hosts defend themselves against viruses. You’ll learn how vaccination is often the best way to protect ourselves against the threat of new and infectious viruses. At the end of the book you’ll find a glossary of scientific terms used, and a list of additional resources. above Virus-infected camellia flowers show a gorgeous red and white variation. Viruses that affect flowercolor are called color-breaking viruses.

WHAT IS A VIRUS

A virologist is a person who studies viruses. Viruses themselves are less easy to define. Virologists have been struggling to find a watertight definition for more than a century. The problem is that every time they think they have found a good definition, someone discovers a virus that doesn’t fit, and the definition has to change. 

The Oxford English Dictionary defines a virus as “an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is in a position to multiply only within the living cells of a number .” 

As a definition, it’s a good start—except that some viruses don’t have a protein coat; others are large enough to be seen in an ordinary light microscope; and some kinds of bacteria are only ready to multiply within a living host cell.

We think of germs as things that make us sick, and that includes both viruses and bacteria, so what’s the difference between bacteria and viruses? Bacteria, in common with other living cells, can generate their own energy, and translate the DNA sequences of their genes into proteins. Viruses can do neither. 

Some giant viruses, discovered quite recently, can make some of the parts needed to translate their genes into proteins, so even this is not a perfect distinction. Viruses continue to be slippery customers. As we discover more about viruses, the definition of a virus will almost certainly change again. 

For the purposes of this book, a virus is an infectious agent that is not a cell, that consists of genetic material in the form of a nucleic acid molecule (DNA, or its cousin, RNA), usually in a protein coat, and is capable of directing its own reproduction and spread by co-opting the machinery inside the host cell it invades.

Above anD below Viruses come in a huge array of shapes, from regular geometric structures to rather amorphous shapes, and a wide range of sizes that vary by about 100-fold. The virus drawings on these two pages are shown to scale.

Viruses come in a wide variety of sizes and shapes. The smallest viruses are about 17 nanometers (nm) in length, where a nanometer  isa millionth of a millimeter. The largest virus   discovered so far is 1,500 nm (or 1.5 micrometers) in length, nearly 100 times the size, and comparable with very small bacteria. For comparison, a human hair is about 20 micrometers across. All but the very largest viruses are too small to see in a  light microscope, and require an electron microscope to visualize.

Earlier definitions of viruses usually included something about disease. At one time it was thought that all viruses cause disease, but now  we know that many viruses don’t. Indeed, some are important and necessary components of their host’s life. Just as we now know that bacteria are an important part of our own, ecosystem viruses have a vital role to play as well.