THE MICROBIOLOGIC SCALE OF PARASITISM
The bacteria, rickettsias, pleuropneumonia-like organisms and viruses are the smallest and simplest known forms of life. These microscopic and ultramicroscopic forms may be arranged in a scale of diminishing size and complexity of organization. This scale in general parallels a scale of diminishing metabolic independence and of increasingly obligatory parasitism. No implication is intended that such an arrangement rests on genetic relationship. No preference is implied either between the hypothesis that the viruses represent primitive forms from which more complex forms of life have evolved76 and the alternative hypothesis that the viruses are degenerate forms which have lost in their parasitic habit the means for their own independent existence.77 Either hypothesis can be supported, but without decisive evidence. Arrangement of these minute forms in a microbiologic scale, even if somewhat artificial, however, does afford a certain rational continuity in the consideration of their
MUDD S. PATHOGENIC BACTERIA, RICKETTSIAS AND VIRUSES AS SHOWN BY THE ELECTRON MICROSCOPETHEIR RELATIONSHIPS TO IMMUNITY AND CHEMOTHERAPY II. RELATIONSHIPS TO IMMUNITY. JAMA. 1944;126(10):632–639. doi:10.1001/jama.1944.82850450003009
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