Infectious diseases are caused by subcellular infectious entities (prions, viruses), prokaryotic bacteria, eukaryoticfungi andprotozoans, metazoanan imals, such as parasitic worms (helminths), and some arthropods. Definitive proof that one of these factors is the cause of a given infection is demon strated by fulfillment of the three Henle-Koch postulates. For technical rea sons, a number of infections cannot fulfill the postulates in their strictest sense as formulated by R. Koch, in these cases a modified form of the pos tulates is applied.