THE IDENTIFICATION of the various types of infections of the upper respiratory tract is difficult, because complete bacteriologic differentiation is not yet possible. Definition through establishment of clinical criteria is even more difficult, since objective signs and subjective symptoms are unreliable guides.
Dingle1 has identified two types of the common cold which are distinct entities: the one, a coryzal type, with an incubation period of one to two days, and the other, a pharyngeal type, with an incubation period of three to nine days. However, he stated that "without knowledge of the incubation periods, the illnesses in a few of the volunteers in each group could not have been differentiated with certainty."
We have not been able to differentiate either spontaneous or induced colds on the basis of clinical characteristics or incubation periods. During the same epidemic, and in the same household, we have noted mild symptoms in some
FOX N, LIVINGSTON GS. VIRUS VACCINE IMMUNIZATION AGAINST THE COMMON COLD. Arch Otolaryngol. 1949;50(4):406–416. doi:10.1001/archotol.1949.00700010418004
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