The second half of this encyclopedic and critical review of the present state of knowledge in this rapidly advancing field of human and comparative medical science is devoted to three major fields, to wit the viruses as agents of infection, immunity against such infections, and the technic of investigation of plant viruses. Dr. Doerr surveys the natural transfers of viruses by germinative, intra-uterine, contact methods and arthropod vectors and the experimental ones by intracerebral, intraneural, intranasal and corneal inoculation and culture on the chorioallantois of the chick embryo. He also reviews the qualitative analysis of viruses by serial passage, determination of specificities, detection of unsuspected latent infections, the establishment of genetic relationships among viruses, the etiologic associations of virus + bacteria and virus + virus, and methods of identification of viruses. The quantitative study of viruses is carried on by titration and the count of colonies of bacteriophage, lesions on the chorio-allantois,