In this chapter, infectious changes to nucleic acids are discussed, including those caused by viruses, plasmids, transposons, and introns. They are grouped in this way because some of them appear to be related, and similar processes are involved when they infect and change genomes. Infections by some viruses are well known for their ability to mutate the host genome (e.g., HIV and human papilloma virus). However, some bacteria and some eukaryotic parasites (e.g., trypanosomes) also have been documented to alter host genomes causing mutation by insertion of sequences into the host genome. Viruses, plasmids, and parasites are the major agents of horizontal gene transfers, i.e., movements of DNA from one species into another. Likewise, transposons and mobile introns have the ability to insert themselves into specic sites of the genomes where they exist, and some introns have the ability to invade other genomes and, thus, are similar to viruses. In fact, some viruses, introns, and transposons have genes that are very similar. The reverse transcriptases of disparate viruses, such as HIV and cauli£ower mosaic virus, some retrotransposons, and some types of introns, all have reverse transcriptases that are related phylogenetically. Additionally, these reverse transcriptases have similarities to telomerases in eukaryotes (Figure 9.1).