The behaviour of three viruses, rinderpest, Newcastle disease, and Rift Yalley fever, in different hosts has been statistically appraised. The population patterns were characterised by a sequence of lag, expansion, peak and decline phases. Increase and decrease in the populations vers exponential. A complete curve was associated with recovery of the infected host. Deaths terminated the sequence but never during the lag phase nor in the early part of the expansion phase. The following statistical comparisons were made with each of the three viruses: (i) between growth curves in different hosts given similar doses of one strain of virus, (ii) between growth curves in on© host given similar doses of different strains of virus, (iii) between growth curves in different tissues of one host given similar doses of on© strain of virus and (iv) between growth curves in one host given different doses of one strain of virus. The following statistical comparisons were made with each of the three viruses: (i) between growth curves in different hosts given similar doses of one strain of virus, (ii) between growth curves in one host given similar doses of different strains of virus, (iii) between growth curves in different tissues of one host given similar doses of on© strain of virus and (iv) between growth curves in one host given different doses of one strain of virus. Two parameters, the rate of increase and the curvature, were found to be constants. They were independent of the strain and dose of virus. They were independent of the host and the tissue sampled. Moreover, only minor differences in the rates occurred between virus species. Two parameters, the length of the lag phase and the peak titre, varied. The former was inversely related to the concentration of infective virus in the dose. The latter was directly related to adaptation to the host.