The smallest are barely larger than ribosomes or other internal structures found inside normal cells; the largest (e.g., variola) are almost the size of small bacteria. Some viruses can be detected under a sophisticated light microscope, but for most only an electron microscope will serve. For generations, therefore, even the best scientists were essentially guessing about the structure, nature, and functioning of viruses, entities they could observe only indirectly (Bookchin and Schumacher 68). The first actual sighting of a variola virus-the first virus of any sort so visualized-came in 1947, under an electron microscope. The smallpox virus capsid is often described as ellipsoid or brick shaped. Many viruses, including variola, are often enveloped, with the capsid housed inside an outer membrane made of proteins, carbohydrates, and lipids, which provides, in the case of variola, an overall spherical appearance (Imperato 390). The infection process begins when a virus approaches a target cell; forcible entry into the cell may then be accomplished in several different ways. Some antibacterial viruses mechanically drill a hole in the cell's outer structure and inject the viral DNA through it, much like the operation of a hypodermic syringe, with the now empty capsid remaining outside. Viruses that afflict plants often enter passively through a microtear in the cell wall (Imperato 390). The defenses that the human body-and modern medicine-erect against many other types of infections are often less effective against viruses. But viruses pose exceptional challenges to the immune system. Some of these aggressors undergo very frequent mutation, altering the external configuration of those glycoproteins, and thereby fooling or evading the antibodies (Strohman 169).
Dear Granny the main problem is that scientists have not yet come to conclusion concerning a question whether viruses are alive or dead. On the one hand, a virus is nature's ultimate parasite: it is incapable, by itself, of undertaking the usual array of biological functions. It cannot produce or consume energy, move, grow, or reproduce without first invading a living cell and usurping the host's internal mechanisms. For this reason, many scientists do not consider variola or any other virus to be truly alive, even in the sense that other simple microorganisms, from bacteria to rickettsia (another family of infectious microorganisms) are considered so (Strohman 169). Other authorities would, perhaps more generously, consider viruses to be minimal "living organisms". At the same time, there is surely something about a virus that makes it different from a conglomeration of inert chemicals. It has at least a kind of life potential, a dormant biological presence, that is undeniable. The recent discovery of categories of even smaller, and even less lifelike, infective entities has further obscured the question of deriving a meaningful definition of life (Strohman 169). Prions, proteinlike particles devoid of any nucleic acid, have been implicated in a variety of degenerative brain diseases, such as scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans, although their precise operational mechanisms are still unknown. Viroids are a cluster of similar plant-invading creatures, containing a snippet of RNAbut none of the other