Dedication: To Professor Hendrik J Koornhof, mentor, friend and man with immense knowledge, insight, wisdom and compassion, who has been a true inspiration to all those who have had the privilege to know him. The planet Earth was formed, in a molten state, some 4.5 billion years ago. It cooled off sufficiently 4 billion years ago to allow the formation of the oceans. Cyanobacteria, found in rock fossil records that are approximately 3.6 billion years old, provide the earliest evidence of life in the form of complex unicellular organisms. Molecular phylogeny is the tool that enables us to understand life in all its complexity and recognise relationships between organisms. In essence, using molecular techniques, we are able to determine the evolutionary relationships of living creatures. By comparing the difference in sequences of homologous genes encoding ribosomal RNA (16S rRNA genes from prokaryotic cells and 18S rRNA from eukaryotic cells) we can measure the evolutionary distance between species of organisms. Computer analysis of rRNA gene sequences suggests that cellular life has evolved along three major lineages. Two of these, Bacteria and Archaea, are exclusively microbial and consist of prokaryotic cells. The third lineage, Eukarya, not only contains unicellular organisms but also myriad multicellular organisms. Two important points have emerged from the study of molecular phylogeny: (i) unicellular organisms are the major and most diverse form of life; and (ii) eukaryotes are not of recent origin, as previously thought, but as ancient as the Bacteria and Archaea lineages, all of which have emerged from a universal ancestor. Although the human race may live in harmony, and is subject to colonisation with many different prokaryotic (e.g. bacteria) and eukaryotic (e.g. fungi and parasites) organisms, this harmony is shattered from time to time when a relatively restricted number of microbial species enter our body and cause pathology-infection.