Studies on the Nature of the Virus of Influenza Ii. Th~ Size of the Infectious Unit in Influenza a by Leslie

The size of the pathogenic agent of influenza A has, until recently, been generally accepted as that of a sphere about 100 m# in diameter. The viruscontaining preparations used in both the ultrafiltration (1) and the centrifugation experiments (2) upon which this estimate was based, consisted of emulsified lung tissue from infected mice. In the preceding paper two of us have shown that particles having an average diameter of 100 m~ can be obtained in essentially equal quantities from both normal and infected lungs (3). The infectious and non-infectious particles showed no significant difference in chemical composition, density, spectral absorption, staining properties, or electron microscopic appearance. This suggested that some particulate element of normal tissues acts as carrier for a considerably smaller infectious agent. Strong support for this theory was obtained when the extra-embryonic fluids of chicks infected with the virus of influenza A were found to contain the virus in high concentration although both normal and infective fluids were essentially free from particles approaching /100 mt~ in diameter (4). I t was found that the virus existed in such a state in the egg fluids that it frequently could not be sedimented at centrifuge speeds which removed the activity almost entirely from mouse lung filtrates. I t was also shown that 100 mu particles separated from normal lungs can absorb the virus almost completely from the egg fluid and thereby acquire the infective property. Thus clear evidence was obtained that normal lung constituents can act as passive carriers of a smaller virus unit. From semiquantitative data obtained with the concentration centrifuge, and from considerations based on the infectivity titers in relation to the total egg fluid protein, it was suggested that the elementary vires particle is probably not much more than 10 m# in diameter (cf. 5).