In this introductory session we shall consider the important micro-organisms, the 'away team', and our host defences, 'the home team'. They existed long before man and we have had to adapt to live in an environment where they are far more abundant. This non-harmful balance exists when the immune system works well, but these same organisms can cause infection when the latter fails. Ticks and mites Mesozoa and leeches As these latter organisms cause infestations rather than infectious disease they will not be further considered in this course. Infection is the invasion or colonisation of the body by pathogenic organisms.
Infectious disease is by far the greatest cause of morbidity and mortality world-wide.
Indeed respiratory infections and gastrointestinal infections cause more deaths world-wide than all other diseases added together. The consequences of infectious disease are protean. No system of the body escapes infection.