ABSTRACT
Th ere are fi ve main classes of pathogens that cause infectious disease. Th ese are viruses, bacteria, fungi, protozoa and worms (also known as helminths). Table 1.1 gives an overview of their size, their structure and the typical diseases they cause. In addition to these pathogens, there are prions, which are not organisms but are proteins that cause infection. A sixth class of organisms are insects that inhabit the skin, known collectively as ectoparasites.
