We tested the hypothesis of optimal adaptation of viral infectors to eukaryotic hosts, using (1) correlation in codon and amino acid usage between organisms, and (2) canonical correlation between groups of hosts and infectors. The codon correlations between parasites and hosts vary, being low between swine and African swine fever virus (ASF; r = 0.18), and highest between potato and potato virus X (r = 0.60). The correlations might indicate different stages of evolution toward optimal adaptation of the parasite codon distribution to the host tRNA pools. The amino acid correlations vary from r = 0.71 between pig and ASF, to 0.88 between catfish and its herpesvirus. It was observed that both in virus and hosts, there is a negative correlation between frequency of an amino acid and molecular weight. Therefore, it was advanced that viral infectors might be preadapted to their hosts because of similarities of the tRNA pools of hosts, and that evolution toward optimization would be dependent on the size of the divergence between the codon distributions of infector and host. Preadaptation does not imply origin of the virus by lateral transfer from the present host, since the correlation of the molecular weight of amino acids with their abundance in proteins is a general phenomenon.