[Evolution and ecological changes of animal viruses].

Although the origin of viruses has not yet been clarified, definite differences in evolutionary patterns have been found among RNA. Retro and DNA viruses. These differences are reflected in infectious diseases. RNA viruses, which have RNA in their genome, replicate many times over in cells within a short period of time, destroying the host cells and causing an acute infection. As the error frequency of RNA replicase enzymes is high, the rate of evolution of RNA viruses is very rapid. Retroviruses also contain RNA as their genome, but the genome RNA is reversely transcribed to the DNA in nuclei and then incorporated into the host chromosome to replicate. The error frequency of reverse transcriptase is also high, and therefore mutations easily occur as well. The transcribed DNA is integrated into host DNA in the nucleus, and it remains in the integrated state for human entire life time, causing chronic disease or developing malignant tumors. As DNA viruses except poxviruses replicate inside the cell nucleus and the error frequency of DNA polymerase is low, the speed of mutation and the degree of resulting diversity are lower than those in the case of the RNA virus. DNA viruses tend to stay inside the body for long periods of time and easily become latent. In this paper, I shall discuss 1) the nature of viruses, 2) the origin of viruses, 3) mutation and recombination, 4) diversity of RNA viruses, 5) quickly changing viral diseases, 6) eradicated viral diseases, 7) chronic and malignant diseases, and 8) control of viral diseases.