Microbiology defined-study of invisible organism: bacteria, viruses, fungi, prions, parasites, protozoa, helminths, arthropods. Prion=protein (each one of us have normal prion proteins, disease causing prions are altered in some way. 54 million deaths globally: 1/3 infectious disease. Worldwide, infectious diseases are responsible for more than 25% or about 15 million deaths each year. Pathogens include bacteria, viruses, fungi, protozoa, parasitic worms, and prions. Microorganism may be within us but it need not cause disease in which case it is not a pathogen, only pathogen if it causes disease (severity of disease is measure of virulence: pneumonia, hiv/aids, diarrheal disease/ tb. Plague (eursinius pestis) , polio, aids, bioterrorism (anthrax: 3000 a year for plague, polio was water borne, fdr. 1867: tb is leading cause of death in canada. 1902: first free sanatorium for tb treatment (muskoka) 1928: provincial special hospital for infantile paralysis opens (edmonton) 1943: first use of penicillin canadian soldiers overseas and last case of small pox.