Although scientists have known since the late nineteenth century that a type of microorganism much smaller than bacteria caused certain diseases, it was not until the 1930's, when scientists were able to crystallize viruses, that viruses were identified as being distinctly different from bacteria, and it was not until the development of the electron microscope that viruses could finally be seen.
Since then, it has been agreed that whereas viruses do fulfill one of the most basic attributes of life, that is reproduction via gene-controlled heredity, they lack the ability to carry out reproduction or even metabolism without the presence of a host cell. Such an ambiguous situation has given rise to the question of whether viruses are truly living organisms. Of the three hypotheses that attempt to answer this question, one holds that viruses have evolved to a most extreme form of parasitism, losing, in the process, all organelles except their nucleus. This hypothesis argues that viruses are a degenerate form descended from fully cellular ancestors and, thus, that viruses should be regarded as living. Another hypothesis believes viruses to the descendants of the "nearly living" stage of early life that existed in the primordial ocean, surviving after the ocean's disappearance by becoming parasites on cellular organisms. A third hypothesis considers viruses to be fragments of genetic material from cellular organisms that evolved the ability to produce an encapsulating protein coat to contain their nucleic acid.
Whatever the nature of their origin, viruses do lack all features of cellular
