What types of infectious agents are there?

Bacteria (prokaryotes) 1-5 micrometers, contain RNA and DNA, can synthesise and metabolise protein, generally free livingMajority are harmless, but a certain small list can cause infectious diseasesOne of diseases with highest disease burden (impact of a disease as measured by e.g. financial cost, mortality, morbidity) is TB (Mycobacterium tuberculosis)Other diseases: bacterial pneumonia, food poisoning, tetanus, typhoid fever, diphtheria, syphilis, leprosy, chlamydia (intracellular) Virus 20-300 nanometers, contain proteins and either RNA or DNA, obligate parasites (cannot reproduce autonomously)Much more likely to be pathogenicSmallpox, influenza, mumps, measles, chickenpox, ebola and rubella Fungi Eukaryotic microbes that normally consume dead organisms (saprophytes); can cause diseases in animals, but mostly affect plants; spore size 1-40 micrometresAspergillosis and candidiasis Parasites Either protozoa (unicellular eukaryotes with a flexible cell membrane) or parasitic worms (helminths, multicellular eukaryotes)Key one is malaria, causes 900,000 deaths/annum, caused by Plasmodium spp.Other diseases include Leishmaniasis, trypanosomiasis, schistosomiasis, Chagas disease Prions Simplest infectious agents, single protein molecule that can catalyse a conformational change in an endogenous protein (PrP) ad infinitumDisease is produced by structural and functional changes that resultScrapie, BSE (mad cow disease) and Creutzfeldt-Jakob disease